Inch to Kilometer Converter
Convert inches to kilometers with our free online length converter.
Quick Answer
1 Inch = 0.0000254 kilometers
Formula: Inch × conversion factor = Kilometer
Use the calculator below for instant, accurate conversions.
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All conversion formulas on UnitsConverter.io have been verified against NIST (National Institute of Standards and Technology) guidelines and international SI standards. Our calculations are accurate to 10 decimal places for standard conversions and use arbitrary precision arithmetic for astronomical units.
Inch to Kilometer Calculator
How to Use the Inch to Kilometer Calculator:
- Enter the value you want to convert in the 'From' field (Inch).
- The converted value in Kilometer will appear automatically in the 'To' field.
- Use the dropdown menus to select different units within the Length category.
- Click the swap button (⇌) to reverse the conversion direction.
How to Convert Inch to Kilometer: Step-by-Step Guide
Converting Inch to Kilometer involves multiplying the value by a specific conversion factor, as shown in the formula below.
Formula:
1 Inch = 2.5400e-5 kilometersExample Calculation:
Convert 10 inches: 10 × 2.5400e-5 = 0.000254 kilometers
Disclaimer: For Reference Only
These conversion results are provided for informational purposes only. While we strive for accuracy, we make no guarantees regarding the precision of these results, especially for conversions involving extremely large or small numbers which may be subject to the inherent limitations of standard computer floating-point arithmetic.
Not for professional use. Results should be verified before use in any critical application. View our Terms of Service for more information.
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View all Length conversions →What is a Inch and a Kilometer?
The inch is a unit of length in the British imperial and United States customary systems of measurement. Since the international yard and pound agreement of 1959, one inch has been defined as exactly 2.54 centimeters. This makes the inch equal to 1/12 of a foot, 1/36 of a yard, and 25.4 millimeters precisely.
The inch is denoted by the symbol in or by a double prime mark ″ (e.g., 5″ means 5 inches). In engineering drawings and construction blueprints, dimensions are typically expressed in feet and inches (e.g., 5′-3″ means 5 feet, 3 inches).
Unlike the metric system with its consistent decimal relationships, the inch exists within a complex system where 12 inches make a foot, 3 feet make a yard, and 1,760 yards make a mile. This fractional system reflects the inch's origins in human-scale measurements rather than mathematical convenience.
The kilometer is a unit of length in the International System of Units (SI), the modern form of the metric system. One kilometer equals exactly 1,000 meters by definition, making it a straightforward decimal multiple of the base SI length unit.
The kilometer is denoted by the symbol km (lowercase 'k', lowercase 'm'). The spelling varies by region: "kilometre" in British English and Commonwealth countries, "kilometer" in American English, though both refer to the same measurement.
The kilometer represents the standard distance unit for:
- Road distances and highway signage worldwide (except USA, Myanmar, Liberia)
- Geographic features: city separation, coastline length, river length
- Transportation: vehicle range, fuel efficiency (km/L or L/100km)
- Athletics: running and cycling race distances (5K, 10K, marathon)
- Aviation: visibility distances, some altitude references in certain countries
The metric system's decimal structure makes kilometer conversions simple:
- 1 kilometer = 1,000 meters (exactly)
- 1 kilometer = 100,000 centimeters
- 1 kilometer = 1,000,000 millimeters
- 1 meter = 0.001 kilometers
- 1 centimeter = 0.00001 kilometers
Note: The Inch is part of the imperial/US customary system, primarily used in the US, UK, and Canada for everyday measurements. The Kilometer belongs to the metric (SI) system.
History of the Inch and Kilometer
The inch has one of the longest continuous histories of any modern measurement unit, with roots stretching back over two millennia.
Ancient Origins: The Roman "uncia" (meaning one-twelfth) represented 1/12 of a Roman foot, similar to our modern inch. When the Romans occupied Britain, they brought this measurement system with them, establishing a tradition that would last far beyond their empire.
Medieval Definitions: Throughout the Middle Ages, various rulers attempted to standardize the inch using body-based measurements. King Edward I of England ordered in 1305 that an inch should equal the length of three barleycorns (grains of barley) laid end to end. King Edward II formalized this in 1324, making it English law. Other definitions included the width of a man's thumb at the base of the nail.
Regional Variations: Before standardization, different regions maintained slightly different inch measurements. The Tower inch, Merchant's inch, and Cloth inch all differed slightly, causing confusion in trade and construction. The French pouce (thumb) was approximately 1.066 English inches.
1959 International Agreement: The modern inch was defined by the international yard and pound agreement signed by the United States, United Kingdom, Canada, Australia, New Zealand, and South Africa. They agreed that 1 yard equals exactly 0.9144 meters, making 1 inch exactly 2.54 centimeters. This ended centuries of slight variations and created perfect mathematical alignment between imperial and metric systems.
US Survey Inch: For a time, the US maintained two slightly different inches—the international inch (2.54 cm exactly) and the US survey inch (2.540005080010160020 cm, based on the Mendenhall Order of 1893). As of January 1, 2023, the US officially retired the survey inch, making all American measurements use the international inch exclusively.
The kilometer emerged during one of history's most ambitious standardization efforts—the creation of the metric system during the French Revolution.
Metric System Origins (1790s): In 1791, the French Academy of Sciences proposed a universal measurement system based on natural constants rather than arbitrary royal decrees. They defined the meter as one ten-millionth of the distance from the North Pole to the equator along the Paris meridian. For longer distances, they created the kilometer (from Greek khilioi = thousand, plus metron = measure).
Decimal Simplicity: Unlike traditional systems with complex conversions (5,280 feet per mile, 1,760 yards per mile), the metric system used consistent decimal relationships. This made the kilometer instantly comprehensible: 1 km = 1,000 m, with no fractional arithmetic required.
Global Adoption: The metric system spread rapidly throughout Europe in the early 1800s. Napoleon's conquests carried metric measurements across the continent. By the mid-19th century, most European nations had officially adopted kilometers for distance measurement.
International Treaty (1875): The Metre Convention, signed by 17 nations, established the International Bureau of Weights and Measures (BIPM) and made the metric system the international scientific standard. Kilometers became the globally recognized unit for geographic and travel distances.
Road Sign Conversion: Through the 20th century, country after country converted road signage from miles to kilometers:
- Germany: 1920s (early adopter)
- Japan: 1924
- France: Kilometers used since metric system creation (1795)
- Australia: 1974 (major conversion effort)
- Canada: 1977 (gradual conversion)
- United Kingdom: Still uses miles despite metric adoption elsewhere
- Ireland: 2005 (one of the last European conversions)
Modern Definition (1983): The meter was redefined based on the speed of light: the distance light travels in vacuum during 1/299,792,458 of a second. This made the kilometer precisely 299,792,458/299,792,458,000 of the distance light travels in one second, tying it to a fundamental physical constant rather than a physical artifact.
Global Standard: Today, approximately 195 of 198 countries use kilometers as their primary road distance measurement. Only the United States, Myanmar, and Liberia officially use miles, though Myanmar is gradually transitioning to metric. The kilometer has become effectively universal for international travel, trade, and communication.
Common Uses and Applications: inches vs kilometers
Explore the typical applications for both Inch (imperial/US) and Kilometer (metric) to understand their common contexts.
Common Uses for inches
Home Construction and Renovation
Residential construction in the United States operates almost exclusively in inches and feet. Standard framing uses 16-inch or 24-inch on-center stud spacing, creating modular dimensions that align with 48-inch-wide drywall and plywood sheets. Contractors calculate materials in board feet and square footage, but individual measurements are precise to 1/16 inch.
Kitchen cabinets come in 3-inch width increments (12″, 15″, 18″, 21″, 24″, 27″, 30″, 33″, 36″), with standard depth of 24 inches for base cabinets and 12 inches for wall cabinets. Countertop height standardizes at 36 inches, while bar counters sit at 42 inches. These dimensions have remained unchanged for decades, creating an entire ecosystem of compatible fixtures, appliances, and hardware.
Tile installation relies on precise inch measurements. Floor tiles commonly measure 12″ × 12″, 12″ × 24″, or 24″ × 24″. Wall tiles might be 3″ × 6″ subway tiles or 4″ × 4″ ceramic squares. Grout line spacing typically runs 1/16″ to 1/4″ depending on tile type. Tile setters work to extremely tight tolerances, as even 1/16-inch variation across a room becomes visibly noticeable.
Manufacturing and Machining
American manufacturing maintains imperial measurements for historical and practical reasons. Machine shops operate in decimal inches (0.001″ = 1 "thou" or 1 "mil"), with precision machining achieving tolerances of ±0.0001″ (one ten-thousandth of an inch, called a "tenth").
Thread specifications use threads per inch (TPI): 1/4″-20 means a 1/4-inch diameter bolt with 20 threads per inch. Common thread pitches include coarse (UNC) and fine (UNF) standards, with sizes ranging from #0-80 (tiny electronic screws) to several inches in diameter for industrial equipment.
Sheet metal thickness traditionally uses gauge numbers (inversely related to thickness in inches). 20-gauge steel is approximately 0.036 inches thick, while 10-gauge is 0.135 inches. However, modern manufacturing increasingly specifies thickness in decimal inches (0.063″ aluminum sheet) for clarity.
CNC (Computer Numerical Control) machines can work in either inches or millimeters, but American shops predominantly program in inches. A machinist might bore a hole to 0.5005″ diameter to accept a 1/2-inch precision shaft with 0.0005″ clearance.
Fasteners and Hardware
The American fastener industry operates entirely in imperial measurements. Screw and bolt diameters are specified in fractions (1/4″, 5/16″, 3/8″, 1/2″) or gauge numbers (#4, #6, #8, #10). Thread pitch is given in threads per inch.
Common bolt sizes and their applications:
- 1/4″-20: Electronics, light fixtures, small brackets
- 5/16″-18: Medium structural connections, automotive
- 3/8″-16: Heavy brackets, machinery, automotive chassis
- 1/2″-13: Structural steel, heavy machinery, trailer hitches
- 5/8″-11: Building construction, large structural connections
- 3/4″-10: Industrial equipment, heavy structural steel
Hex keys (Allen wrenches) come in fractional inch sizes: 1/16″, 5/64″, 3/32″, 1/8″, 5/32″, 3/16″, 7/32″, 1/4″, 5/16″, and 3/8″. Metric hex keys are incompatible—a 5mm key is 0.197″, close to but not quite matching 3/16″ (0.1875″) or 1/4″ (0.25″).
Display Technology and Photography
Television and monitor sizes are universally described by screen diagonal in inches, even in metric countries. This convention originated with cathode ray tubes, where the diagonal measurement corresponded to the visible screen area inside the bezel.
Common display sizes and their uses:
- 24″ monitors: Standard office workstation
- 27″ monitors: Popular desktop size, ideal for 1440p resolution
- 32″ monitors: Large desktop or small TV
- 43″ TVs: Small bedroom TV
- 55″ TVs: Most popular living room size
- 65″ TVs: Premium living room standard
- 75″ TVs: Home theater
- 85″+ TVs: Luxury home cinema
Photography inherited imperial measurements from early camera equipment. Sensor sizes reference inches in confusing ways—a "1-inch sensor" is actually much smaller (13.2 × 8.8 mm), referencing the outer diameter of an obsolete video camera tube rather than the actual sensor dimensions.
Large format photography uses inch measurements directly: 4×5″ and 8×10″ sheet film, or medium format 6×4.5cm, 6×6cm, 6×7cm (which are actually metric despite being called 120 film).
Plumbing and Pipe Systems
Plumbing pipe sizes in the US use nominal measurements that don't match actual dimensions. A "1/2-inch pipe" has an inside diameter of approximately 0.622″ and outside diameter of 0.840″. This naming convention dates back to when pipe walls were much thicker.
Common pipe sizes and uses:
- 1/2″ nominal: Standard water supply lines to fixtures
- 3/4″ nominal: Main water supply within house
- 1″ nominal: Supply from water meter to house
- 1½″ nominal: Sink drain lines
- 2″ nominal: Toilet drain, main stack
- 3″ nominal: Main drain, sewer connection
- 4″ nominal: Main building drain and sewer
Copper pipe uses different sizing: Type M, L, and K copper all have the same outside diameter for a given size, but different wall thicknesses. PEX (cross-linked polyethylene) tubing uses actual measurements—1/2″ PEX has a 0.500″ outside diameter.
Weather Reporting and Climatology
United States weather forecasting measures precipitation in inches. The National Weather Service issues warnings based on inch thresholds:
- Flash Flood Watch: Expected 1-2 inches in one hour or 3-4 inches in several hours
- Heavy Snow Warning: 6+ inches in 12 hours or 8+ inches in 24 hours
- Blizzard Warning: 3+ hours of heavy snow with sustained winds 35+ mph
Rainfall intensity rates:
- Light rain: <0.10 inches per hour
- Moderate rain: 0.10 to 0.30 inches per hour
- Heavy rain: >0.30 inches per hour
Annual precipitation varies dramatically across the US:
- Las Vegas, NV: 4.2 inches per year (desert)
- Phoenix, AZ: 8.3 inches per year
- Los Angeles, CA: 14.9 inches per year
- New York, NY: 46.2 inches per year
- Seattle, WA: 37.5 inches per year
- Miami, FL: 61.9 inches per year
- Mobile, AL: 67.0 inches per year (wettest major US city)
- Mt. Waialeale, HI: 450+ inches per year (one of Earth's wettest places)
Automotive and Cycling
American automotive specifications traditionally use inches:
- Wheel Diameter: 15″, 16″, 17″, 18″, 19″, 20″, 22″ (passenger vehicles)
- Tire Width: Given in millimeters, but wheel size in inches (e.g., 225/65R17)
- Ground Clearance: Often stated in inches (e.g., 8.6″ clearance)
- Cargo Space: Cubic feet, with dimensions in inches
- Engine Displacement: Historically cubic inches (350 cu in = 5.7L), now usually liters
Bicycle sizing uses a mix of systems. Mountain bike wheels measure 26″, 27.5″ (650B), or 29″ in diameter. Road bikes use the 700c standard (approximately 27″ diameter, derived from French sizing). Frame sizes might be given in inches (17″ mountain bike) or centimeters (56cm road bike) depending on manufacturer and tradition.
HVAC and Duct Work
Heating, ventilation, and air conditioning (HVAC) systems in the US specify duct sizes in inches. Rectangular ductwork might be 6″ × 10″, 8″ × 12″, or 10″ × 16″. Round ducts come in 4″, 6″, 8″, 10″, and 12″ diameters.
Air register and vent sizes are standardized in inches:
- Floor registers: 4″ × 10″, 4″ × 12″, 4″ × 14″
- Wall registers: 6″ × 10″, 6″ × 12″, 8″ × 10″
- Return air grilles: 14″ × 24″, 16″ × 20″, 20″ × 20″
Furnace filters use inch sizing: 16″ × 20″ × 1″, 16″ × 25″ × 1″, 20″ × 25″ × 4″. The first two dimensions are the face size, and the third is the thickness. Standard filters are 1″ thick, while pleated media filters might be 4″ or 5″ thick for better filtration and airflow.
When to Use kilometers
International Road Travel and GPS
GPS navigation systems worldwide default to kilometers in metric countries. Drivers receive instructions like "In 2 kilometers, turn left" or "Your destination is 45 kilometers away." Trip computers display "distance to empty" in kilometers, helping drivers plan fuel stops.
Road atlases and mapping applications show distance scales in kilometers. Drivers estimate travel time using kilometers: "It's 300 km, so at 100 km/h average, that's 3 hours of driving" becomes intuitive mental math.
Highway interchanges number by kilometer markers in many countries. "Exit 245" means the exit is 245 km from the highway's starting point, making distance calculation simple: the gap between Exit 245 and Exit 310 is 65 km.
Public Transportation and Urban Transit
Metro, train, and bus systems describe route lengths and network extent in kilometers:
Urban Rail Systems:
- Tokyo Metro: 304 km of track, 179 stations
- London Underground: 402 km of track
- New York City Subway: 380 km of track
- Paris Métro: 226 km of track
High-Speed Rail:
- China high-speed network: 40,000+ km (world's largest)
- Europe high-speed: 11,000+ km network
- Japan Shinkansen: 3,000+ km
- Trains operate at 250-350 km/h
Commuters describe their journey: "I take the train 35 km to work" or "The metro line is 25 km end to end."
Logistics and Freight
Shipping companies calculate costs, routes, and delivery times in kilometers:
Trucking:
- Per-kilometer rates: €0.50-2.00 per km depending on cargo
- Driver limits: 500-900 km daily maximum (varying by regulations)
- Route optimization: Software minimizes total kilometers driven
Delivery Services:
- Local delivery: 0-50 km radius
- Regional delivery: 50-200 km
- National delivery: 200-1,000+ km
Supply Chain: "Warehouse is 150 km from port" or "Distribution center serves 200 km radius" inform logistics planning and inventory positioning.
Scientific and Environmental Research
Scientific studies report distances in kilometers:
Climate Science:
- Ice shelf extent: "Antarctic ice shelf extends 500 km from coast"
- Glacier retreat: "Glacier receded 5 km over past decade"
- Ocean currents: "Gulf Stream flows 10,000 km across Atlantic"
Geology:
- Fault lines: "San Andreas Fault extends 1,200 km through California"
- Volcanic reach: "Ash cloud spread 500 km from eruption"
Ecology:
- Animal migration: "Caribou migrate 5,000 km annually"
- Bird migration: "Arctic terns migrate 70,000 km round trip"
Sports and Recreation
Beyond running, many sports use kilometer measurements:
Hiking:
- Day hike: 5-20 km round trip
- Multi-day trek: 10-30 km per day
- Long-distance trails: Appalachian Trail 3,500 km, Pacific Crest Trail 4,300 km
Swimming:
- Open water races: 5 km, 10 km, 25 km
- English Channel: 34 km minimum straight-line distance (actual swim 40-50 km due to currents)
Skiing:
- Cross-country ski race: 10 km, 30 km, 50 km distances
- Ski resort terrain: "Resort has 150 km of marked runs"
Sailing:
- Yacht races: Sydney to Hobart 1,170 km, Volvo Ocean Race circumnavigates 72,000+ km
Agriculture and Land Management
Farms and land parcels measure in hectares, with distances in kilometers:
Farm Infrastructure:
- Irrigation canal: 10-50 km length serving agricultural region
- Fence line: "Property has 15 km of fencing"
- Farm roads: "30 km of internal roads"
Rural Distances:
- "Town is 25 km away"
- "Nearest hospital 60 km"
- "Property borders 2 km of river frontage"
Real Estate and Property
Property descriptions include kilometer proximity to amenities:
Location Descriptions:
- "5 km to city center"
- "2 km to nearest school"
- "15 km to international airport"
- "Within 1 km of public transport"
Commute Distance: Property values often correlate with kilometer distance from employment centers. "Within 10 km of downtown" commands premium pricing compared to "40 km from city center."
Emergency Services
Police, fire, and ambulance services track response distances and coverage areas in kilometers:
Response Zones:
- Ambulance coverage: Aim for < 8 km from any location
- Fire station spacing: 3-5 km apart in urban areas
- Police patrol areas: Officers cover 10-30 km² sectors
Response Times: "Average response time 8 minutes for locations within 5 km of station" guides emergency service planning.
Telecommunications and Utilities
Infrastructure planners measure network reach and service areas in kilometers:
Power Distribution:
- High-voltage transmission: Hundreds to thousands of km
- Distribution lines: 5-20 km from substation to customers
- Rural electrification: "Extended power 50 km to remote village"
Water Supply:
- Aqueduct: "200 km pipeline from reservoir to city"
- Distribution network: "City has 2,000 km of water mains"
Internet Infrastructure:
- "Fiber backbone runs 5,000 km across country"
- "Last-mile connections within 3 km of exchange"
Additional Unit Information
About Inch (in)
How many inches are in a foot?
There are exactly 12 inches in one foot. This relationship is fixed by definition in the US customary and imperial systems. The division of a foot into 12 parts dates back to ancient Roman measurements and remained standard because 12 is easily divisible by 2, 3, 4, and 6—making fractional measurements convenient in practical applications.
To convert feet to inches, multiply by 12. To convert inches to feet, divide by 12.
- 5 feet = 5 × 12 = 60 inches
- 48 inches = 48 ÷ 12 = 4 feet
- 3 feet 7 inches = (3 × 12) + 7 = 43 inches total
How many centimeters are in an inch?
One inch equals exactly 2.54 centimeters. This relationship was established by the 1959 international yard and pound agreement, which defined the yard as exactly 0.9144 meters. Since one inch equals 1/36 of a yard, it works out to precisely 2.54 cm.
This is one of the few perfectly defined relationships between imperial and metric systems. There's no rounding or approximation—it's exactly 2.54000... cm per inch by international agreement.
To convert inches to centimeters, multiply by 2.54. To convert centimeters to inches, divide by 2.54.
- 10 inches = 10 × 2.54 = 25.4 cm
- 50 cm = 50 ÷ 2.54 = 19.685 inches
Why does the US still use inches instead of the metric system?
The United States remains one of only three countries (along with Myanmar and Liberia) not officially using the metric system, largely due to historical momentum, infrastructure costs, and cultural identity.
Infrastructure Investment: The US has trillions of dollars invested in imperial-based infrastructure—road signs showing miles, construction materials in feet and inches, manufacturing equipment calibrated in thousandths of an inch, pipelines in inch diameters, and billions of blueprints in feet-inches notation. Converting all of this would cost an estimated $370 billion to $1 trillion.
Industry Standardization: American manufacturing, construction, and engineering have perfected systems around imperial measurements over centuries. Millions of workers are trained in inches and feet. Changing would require retraining entire industries and replacing countless tools, gauges, and documentation.
Cultural Resistance: Americans intuitively understand inches, feet, and miles from childhood. Polls consistently show that most Americans oppose mandatory metric conversion, viewing it as unnecessary government overreach. Unlike temperature (where Fahrenheit provides finer granularity for weather), length measurements in inches don't provide any practical advantage—but changing feels like abandoning tradition.
Failed Conversion Attempts: Congress passed the Metric Conversion Act in 1975, making metric the "preferred system" but keeping adoption voluntary. Without enforcement, industries continued using imperial. A 1982 push to convert road signs to kilometers was abandoned after fierce public backlash.
Partial Adoption: The US actually uses metric extensively in science, medicine, military, and some industries. All US customary units are now legally defined in terms of metric units. Americans live in a hybrid system—drinking 2-liter sodas while driving 65 miles per hour.
What's the difference between decimal inches and fractional inches?
Fractional inches express measurements as common fractions: 1/2″, 3/4″, 5/8″, 7/16″, 15/32″. This system dominates carpentry, construction, and mechanical work. Rulers and tape measures show fraction markings—typically down to 1/16″ for standard use and 1/32″ for precision carpentry.
Decimal inches express measurements as decimal values: 0.5″, 0.75″, 0.625″, 0.4375″, 0.46875″. Engineering drawings, machining, and manufacturing prefer decimals because they're easier to calculate with, especially for tolerances and fits.
Both systems describe the same measurements:
- 1/2″ = 0.500″
- 3/4″ = 0.750″
- 5/8″ = 0.625″
- 1/8″ = 0.125″
Machine shops often work in "thousandths" (0.001″, called a "thou" or "mil") for precision. A machinist might drill a 0.257″ diameter hole—trying to express this as a fraction (257/1000″) would be impractical.
Carpenters and builders work in fractions because tape measures show fractions, and because halving measurements repeatedly (1/2, 1/4, 1/8, 1/16, 1/32) is natural in layout work.
How accurate is an inch measurement?
The definition of an inch is exact: 2.54 centimeters precisely, or 25.4 millimeters. There's no uncertainty in the standard itself.
Practical accuracy depends on the measuring tool:
- Ruler or tape measure: Accurate to roughly ±1/16″ (±1.6 mm) for careful work
- Steel ruler with machinist's markings: ±1/32″ (±0.8 mm) when read carefully
- Dial calipers: ±0.001″ (±0.025 mm) typical accuracy
- Digital calipers: ±0.0005″ (±0.013 mm) for quality instruments
- Micrometers: ±0.0001″ (±0.0025 mm) for precision tools
- Coordinate measuring machines (CMM): ±0.00005″ (±0.0013 mm) or better
Different trades require different precision:
- House framing: ±1/8″ is usually acceptable
- Finish carpentry: ±1/16″ or better for visible joints
- Cabinetmaking: ±1/32″ for tight-fitting doors and drawers
- Machining: ±0.001″ (one thousandth) for standard fits
- Precision machining: ±0.0001″ (one ten-thousandth, called a "tenth")
- Gauge blocks (calibration standards): Accurate to millionths of an inch
Can I use a metric wrench on an inch bolt?
Generally no—metric and imperial fasteners are incompatible. While some sizes appear close, using the wrong wrench damages bolt heads and nuts.
Why they're incompatible: Metric sockets and wrenches are sized in millimeters (8mm, 10mm, 13mm, 17mm), while imperial fasteners use fractional inches (5/16″, 3/8″, 1/2″, 5/8″). These don't align:
- 13mm = 0.512″ (close to 1/2″ = 0.500″, but not exact)
- 10mm = 0.394″ (between 3/8″ = 0.375″ and 7/16″ = 0.438″)
The result: A 13mm wrench on a 1/2″ bolt will be 0.012″ too large—enough to round off the hex corners with sufficient force. A 10mm wrench on a 3/8″ bolt won't fit at all.
Exception: In an emergency, you might carefully use a slightly large adjustable wrench or locking pliers, but this risks damaging the fastener. Always use the correct system—if you work on American-made equipment, you need inch tools. For European or Japanese equipment, you need metric tools.
Many professional mechanics maintain complete sets of both imperial and metric sockets, wrenches, and hex keys—a significant investment but essential for working on diverse equipment.
What does the ″ symbol mean?
The double prime symbol ″ represents inches in technical drawings, blueprints, and mathematical contexts. A single prime ′ represents feet.
Standard notation:
- 5′ = five feet
- 5″ = five inches
- 5′-3″ = five feet, three inches
- 5′-3½″ = five feet, three and a half inches
Origin: These symbols come from astronomical and geometric notation, where they represented divisions of degrees (°). A degree divided by 60 gives minutes (′), and a minute divided by 60 gives seconds (″). This notation was borrowed for feet and inches because of the similar subdividing relationship.
Common substitution: Because typewriters and keyboards lack easy access to proper prime symbols, people often substitute:
- Apostrophe (') for feet: 5' tall
- Quotation mark (") for inches: 5" wide
- Both together: 5'-3" tall
In professional CAD drawings and blueprints, proper prime symbols (′ and ″) are standard. In casual writing, apostrophes and quotes are acceptable and universally understood.
How is screen size measured in inches?
Display screen sizes (TVs, monitors, tablets, smartphones) are always measured diagonally from one corner to the opposite corner, measuring only the visible screen area (not the bezel or frame).
Why diagonal? This convention originated with cathode ray tubes (CRTs), where the diagonal measurement corresponded to the size of the round tube face. When rectangular screens became standard, the diagonal measurement convention persisted for consistency.
Practical dimensions for common sizes:
TVs (16:9 aspect ratio):
- 55″ diagonal = approximately 48″ wide × 27″ tall
- 65″ diagonal = approximately 56.7″ wide × 31.9″ tall
- 75″ diagonal = approximately 65.4″ wide × 36.8″ tall
Monitors (16:9 aspect ratio):
- 24″ diagonal = approximately 20.9″ wide × 11.8″ tall
- 27″ diagonal = approximately 23.5″ wide × 13.2″ tall
Why this matters: When planning where to place a TV or monitor, you need the actual width and height, not the diagonal. Always check specifications for exact dimensions. Also add 1-2 inches in each direction to account for the bezel.
How do I convert inches to millimeters?
To convert inches to millimeters, multiply by 25.4. This gives you an exact result because the inch is legally defined as exactly 2.54 centimeters (25.4 millimeters).
Formula: millimeters = inches × 25.4
Examples:
- 1 inch = 1 × 25.4 = 25.4 mm
- 2 inches = 2 × 25.4 = 50.8 mm
- 0.5 inches = 0.5 × 25.4 = 12.7 mm
- 10 inches = 10 × 25.4 = 254 mm
- 3.25 inches = 3.25 × 25.4 = 82.55 mm
Converting fractional inches: First convert the fraction to a decimal, then multiply by 25.4:
- 1/2″ = 0.5″ = 0.5 × 25.4 = 12.7 mm
- 3/4″ = 0.75″ = 0.75 × 25.4 = 19.05 mm
- 5/8″ = 0.625″ = 0.625 × 25.4 = 15.875 mm
- 1/8″ = 0.125″ = 0.125 × 25.4 = 3.175 mm
Reverse conversion (mm to inches): Divide millimeters by 25.4:
- 50 mm = 50 ÷ 25.4 = 1.969 inches (approximately 2 inches)
- 100 mm = 100 ÷ 25.4 = 3.937 inches (approximately 4 inches)
For quick mental estimates, remember that 25mm ≈ 1 inch. This gives you approximately correct values for rough planning, though it slightly underestimates (25mm is actually 0.984 inches).
Why are lumber dimensions not the actual size?
Lumber in the United States is sold using nominal dimensions—traditional names that don't match the actual milled size. A "2×4" is actually 1.5″ × 3.5″, not 2″ × 4″.
Historical reason: Originally, rough-cut lumber fresh from the sawmill did measure 2″ × 4″. However, the lumber then went through drying (removing moisture) and planing (smoothing surfaces), which reduced dimensions by approximately 1/4″ on each face. Rather than rename everything, the industry kept nominal sizing for ordering convenience while standardizing actual dimensions.
Standard actual dimensions:
- 2×4 → 1.5″ × 3.5″
- 2×6 → 1.5″ × 5.5″
- 2×8 → 1.5″ × 7.25″
- 2×10 → 1.5″ × 9.25″
- 2×12 → 1.5″ × 11.25″
- 4×4 → 3.5″ × 3.5″
- 1×4 → 0.75″ × 3.5″
- 1×6 → 0.75″ × 5.5″
These dimensions are mandated by the American Softwood Lumber Standard (PS 20-20) and are consistent across all retailers.
Why this system persists: Construction is designed around nominal dimensions. Studs are spaced "16 inches on center" in walls, but everyone understands the actual stud is 1.5″ thick. Changing to actual dimension naming would require rewriting building codes, retraining millions of workers, and changing century-old industry conventions.
Important for DIY: Always design projects using actual lumber dimensions, not nominal. If you're building something that needs to be exactly 4 inches wide, a 2×4 laid flat won't work—it's only 3.5 inches.
What is the smallest measurement on a standard tape measure?
Most standard tape measures used in construction and carpentry show markings down to 1/16 inch as the smallest division. Each inch is divided into 16 equal parts, with various line lengths to indicate fractions:
- Longest lines: Every inch (1″, 2″, 3″...)
- Next longest: Half inches (1/2″, 1½″, 2½″...)
- Medium lines: Quarter inches (1/4″, 3/4″, 1¼″...)
- Shorter lines: Eighths (1/8″, 3/8″, 5/8″, 7/8″)
- Shortest lines: Sixteenths (1/16″, 3/16″, 5/16″... through 15/16″)
Precision tape measures for finish carpentry may show 1/32-inch divisions, doubling the number of marks. These are harder to read but provide twice the precision for fine woodworking.
Reading tape measures: The marks use a graduated system where longer marks indicate simpler fractions. The 1/2″ mark is the longest subdivision line, the 1/4″ and 3/4″ marks are slightly shorter, the 1/8″ marks are shorter still, and the 1/16″ marks are the shortest. This visual hierarchy makes it easier to count marks and identify positions quickly without having to count every line.
Practical accuracy: Even with 1/16″ markings, human error in marking and cutting typically limits practical accuracy to about ±1/16″ for construction work. Finish carpentry might achieve ±1/32″ with careful measurement and sharp marking.
For measurements more precise than 1/32 inch, professionals use dial calipers (readable to 0.001″) or digital calipers rather than tape measures.
About Kilometer (km)
How many meters are in a kilometer?
There are exactly 1,000 meters in 1 kilometer. This relationship is definitional—the prefix "kilo-" means 1,000 in the metric system.
Converting kilometers to meters: Multiply by 1,000
- 1 km = 1,000 m
- 2.5 km = 2,500 m
- 0.5 km = 500 m
- 10 km = 10,000 m
Converting meters to kilometers: Divide by 1,000
- 1,500 m = 1.5 km
- 5,000 m = 5 km
- 500 m = 0.5 km
- 42,195 m = 42.195 km (marathon distance)
The metric system's decimal structure makes these conversions simple—just move the decimal point three places.
How many kilometers are in a mile?
One mile equals exactly 1.609344 kilometers. This conversion factor was established through the 1959 international yard and pound agreement, which defined the yard (and thus the mile) in terms of meters.
Quick conversions:
- 1 mile = 1.609 km
- 5 miles = 8.047 km
- 10 miles = 16.093 km
- 26.2 miles (marathon) = 42.195 km
Reverse conversion: 1 kilometer = 0.621371 miles
- 1 km ≈ 0.62 miles (about 5/8 of a mile)
- 5 km = 3.107 miles
- 10 km = 6.214 miles
- 100 km = 62.137 miles
Mental approximation: For rough estimates, remember "5 miles ≈ 8 km" or "8 km ≈ 5 miles." This Fibonacci-pair approximation gives about 1% accuracy and is easy to remember. More roughly, "1 mile ≈ 1.6 km" works for casual conversion.
What does 'kilo' mean in kilometer?
The prefix "kilo-" means one thousand (1,000). It derives from the Greek word khilioi, meaning thousand.
In the metric system, "kilo-" consistently indicates multiplication by 1,000:
- 1 kilometer (km) = 1,000 meters
- 1 kilogram (kg) = 1,000 grams
- 1 kiloliter (kL) = 1,000 liters
- 1 kilobyte (kB) = 1,000 bytes (in decimal notation)
- 1 kilowatt (kW) = 1,000 watts
This consistent prefix system makes the metric system easy to learn. Once you know "kilo = 1,000," you can understand any kilo- measurement immediately.
Other common metric prefixes:
- Mega- (M) = 1,000,000 (million)
- Kilo- (k) = 1,000 (thousand)
- Hecto- (h) = 100 (hundred)
- Deca- (da) = 10 (ten)
- Deci- (d) = 0.1 (tenth)
- Centi- (c) = 0.01 (hundredth)
- Milli- (m) = 0.001 (thousandth)
Is a kilometer longer or shorter than a mile?
A kilometer is shorter than a mile. One mile equals approximately 1.61 kilometers, making a mile about 61% longer than a kilometer.
Comparison:
- 1 mile = 1.609344 km
- 1 km = 0.621371 miles
Practical comparison:
- 5K race = 3.11 miles (shorter than 5 miles)
- 10K race = 6.21 miles (shorter than 10 miles)
- 100 km = 62.1 miles (significantly shorter than 100 miles)
Mental shortcut: Think "kilometers are smaller, so you need more of them." To cover the same distance:
- 100 miles requires 161 kilometers
- 100 kilometers equals only 62 miles
This difference matters when traveling internationally. A road sign showing "Paris 100 km" means about 62 miles—much closer than "100 miles" would be. Speed limits work the same way: "100 km/h" equals about 62 mph, considerably slower than "100 mph."
Why does most of the world use kilometers instead of miles?
Approximately 195 of 198 countries use kilometers because the metric system is simpler, more logical, and internationally standardized. Only the United States, Myanmar, and Liberia officially use miles, with Myanmar gradually transitioning to metric.
Decimal Simplicity: The metric system uses consistent base-10 relationships:
- 1 km = 1,000 m (easy conversion)
- Compare to: 1 mile = 1,760 yards = 5,280 feet (complex conversion)
Children in metric countries learn one simple rule: move the decimal point. American children must memorize: 12 inches per foot, 3 feet per yard, 1,760 yards per mile.
International Trade: The Metre Convention (1875) established metric as the international scientific and commercial standard. Countries adopting metric gained advantages in international trade, scientific collaboration, and technical documentation.
Post-Colonial Transitions: Former British colonies gradually converted from imperial to metric:
- Australia: 1970s
- Canada: 1970s-1980s (though some imperial usage persists)
- India: 1950s-1960s
- South Africa: 1970s
Only the United Kingdom retained miles for road signage despite otherwise adopting metric.
US Resistance: America resisted conversion due to:
- Massive infrastructure costs (replacing millions of road signs)
- Public opposition (polls show Americans prefer familiar units)
- No perceived benefit justifying disruption and expense
- "Metric Conversion Act" of 1975 made metric voluntary, not mandatory
Scientific Reality: Even the US uses metric in science, medicine, and military. The paradox: US scientists publish in metric while consumers shop in imperial.
How do you convert speed from mph to km/h?
To convert miles per hour (mph) to kilometers per hour (km/h), multiply by 1.609344. This gives exact results.
Formula: km/h = mph × 1.609344
Common speed conversions:
- 30 mph = 48.3 km/h (urban speed limit)
- 55 mph = 88.5 km/h (rural highway)
- 60 mph = 96.6 km/h (≈100 km/h common highway limit)
- 70 mph = 112.7 km/h
- 75 mph = 120.7 km/h
- 80 mph = 128.7 km/h
Reverse conversion (km/h to mph): Divide by 1.609344, or multiply by 0.621371
- 50 km/h = 31.1 mph (urban limit in metric countries)
- 90 km/h = 55.9 mph (rural road)
- 100 km/h = 62.1 mph (common highway limit)
- 110 km/h = 68.3 mph
- 120 km/h = 74.6 mph
- 130 km/h = 80.8 mph (German autobahn advisory speed)
Mental approximation: Multiply mph by 1.6 for quick estimates:
- 60 mph × 1.6 ≈ 96 km/h (actual: 96.6, very close)
- 70 mph × 1.6 ≈ 112 km/h (actual: 112.7, very close)
Or divide km/h by 1.6 for reverse estimate:
- 100 km/h ÷ 1.6 ≈ 62.5 mph (actual: 62.1, close enough)
Why it matters: Tourists driving in foreign countries must convert speed limits mentally to avoid speeding tickets. Rental cars may show speedometers in only one unit, requiring constant mental conversion.
What's the difference between km and km²?
Kilometers (km) measure linear distance in one dimension—how far apart two points are, or how long something stretches.
Square kilometers (km²) measure area in two dimensions—how much surface space something covers.
Linear distance (km):
- Road distance: "Paris is 100 km from here"
- Running race: "10 km race"
- River length: "Danube River is 2,850 km long"
- Border: "3,000 km border with neighboring country"
Area (km²):
- City size: "Paris covers 105 km²"
- Country size: "France has an area of 640,000 km²"
- Lake surface: "Lake Geneva covers 580 km²"
- Forest: "National park protects 1,200 km² of wilderness"
The math:
- Multiplying two distances creates area: 10 km × 5 km = 50 km²
- 1 km² = 1,000,000 m² = 100 hectares
- A square with 1 km sides has area of 1 km²
- A square with 10 km sides has area of 100 km² (not 10 km!)
Common confusion: "The city is 50 km" is incomplete—50 km in which direction? Better: "The city center is 50 km away" (linear distance) or "The city covers 200 km²" (area).
How far is a 5K race in miles?
A 5K race is 3.107 miles. The "K" in "5K" stands for kilometers, so 5K means 5 kilometers.
Exact conversion: 5 km × 0.621371 miles/km = 3.10686 miles ≈ 3.11 miles
For practical purposes: 5K ≈ 3.1 miles (slightly over 3 miles)
Common running distances:
- 5K: 5 km = 3.11 miles (popular beginner race distance)
- 10K: 10 km = 6.21 miles (popular intermediate distance)
- 15K: 15 km = 9.32 miles
- Half-marathon: 21.0975 km = 13.11 miles (exactly half of marathon)
- Marathon: 42.195 km = 26.219 miles
- 50K: 50 km = 31.07 miles (ultramarathon)
- 100K: 100 km = 62.14 miles (ultramarathon)
Why "K" notation? Running races worldwide use kilometer distances even in countries that otherwise use miles (like the US). This creates universal race standards—a 5K is the same distance whether run in New York, Paris, or Tokyo. The "K" abbreviation became standard running terminology.
Pace conversion: A runner completing 5K in 25 minutes averages:
- 5:00 per kilometer (25 minutes ÷ 5 km)
- 8:03 per mile (25 minutes ÷ 3.107 miles)
How do you calculate distance on a map with a kilometer scale?
Map scales show how many kilometers in the real world correspond to a unit distance on the map. Common map scales for road maps use kilometers.
Common scale notations:
- 1:100,000 means 1 cm on map = 100,000 cm (1 km) in reality
- 1:250,000 means 1 cm on map = 250,000 cm (2.5 km) in reality
- 1:50,000 means 1 cm on map = 50,000 cm (0.5 km or 500 m) in reality
Using the graphic scale: Most maps include a graphic scale bar showing distances directly:
|——————|
0 50 100 km
Measuring distance:
- Use a ruler or string to measure the map distance
- Compare to the scale bar or use the ratio
- Calculate actual distance
Example: On a 1:100,000 scale map:
- Measure 5 cm between two cities
- 5 cm × 1 km/cm = 5 km actual distance
For curved routes (roads, rivers): Lay a string along the curved path, then measure the string length against the scale.
Digital maps: Online mapping tools calculate distances automatically, displaying both kilometers and miles. Google Maps, for instance, shows "15 km" or "9.3 mi" depending on regional settings.
What's the origin of the marathon distance 42.195 kilometers?
The marathon distance of 42.195 kilometers (26 miles, 385 yards) has a fascinating history combining ancient legend with modern royal preference.
Ancient Legend: The marathon commemorates the run of Greek soldier Pheidippides, who allegedly ran from the Battle of Marathon to Athens (about 40 km) to announce victory over Persia in 490 BCE, then died from exhaustion. This legend inspired the modern marathon race.
Original Olympic Distance (1896-1908): Early Olympic marathons varied in distance, roughly 40 km but not standardized. Different Olympic marathons measured 40 km (1896 Athens), 40.26 km (1900 Paris), 41.86 km (1904 St. Louis), 40 km (1906 Athens).
1908 London Olympics—The Defining Race: The modern distance was set for the 1908 London Olympics. The race began at Windsor Castle and finished at the Olympic Stadium. Originally planned as 26 miles, an extra 385 yards was added so the race would finish directly in front of the royal box where Queen Alexandra sat.
Total distance: 26 miles + 385 yards = 26.219 miles = 42.195 kilometers
Standardization (1921): The International Amateur Athletic Federation (IAAF) officially adopted 42.195 km as the standard marathon distance in 1921, codifying the 1908 London course distance for all future marathons.
Modern Usage: Every certified marathon worldwide measures exactly 42.195 kilometers (or 26 miles 385 yards). Famous marathons—Boston, New York City, London, Berlin, Tokyo—all use this precise distance. Race courses must be certified to ensure accuracy, typically measured using calibrated bicycle wheels.
Half-Marathon: Exactly half of 42.195 km = 21.0975 km (13.1 miles), also standardized for half-marathon races worldwide.
Conversion Table: Inch to Kilometer
| Inch (in) | Kilometer (km) |
|---|---|
| 0.5 | 0 |
| 1 | 0 |
| 1.5 | 0 |
| 2 | 0 |
| 5 | 0 |
| 10 | 0 |
| 25 | 0.001 |
| 50 | 0.001 |
| 100 | 0.003 |
| 250 | 0.006 |
| 500 | 0.013 |
| 1,000 | 0.025 |
People Also Ask
How do I convert Inch to Kilometer?
To convert Inch to Kilometer, enter the value in Inch in the calculator above. The conversion will happen automatically. Use our free online converter for instant and accurate results. You can also visit our length converter page to convert between other units in this category.
Learn more →What is the conversion factor from Inch to Kilometer?
The conversion factor depends on the specific relationship between Inch and Kilometer. You can find the exact conversion formula and factor on this page. Our calculator handles all calculations automatically. See the conversion table above for common values.
Can I convert Kilometer back to Inch?
Yes! You can easily convert Kilometer back to Inch by using the swap button (⇌) in the calculator above, or by visiting our Kilometer to Inch converter page. You can also explore other length conversions on our category page.
Learn more →What are common uses for Inch and Kilometer?
Inch and Kilometer are both standard units used in length measurements. They are commonly used in various applications including engineering, construction, cooking, and scientific research. Browse our length converter for more conversion options.
For more length conversion questions, visit our FAQ page or explore our conversion guides.
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📚 How to Convert Units
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🔢 Conversion Formulas
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⚖️ Metric vs Imperial
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⚠️ Common Mistakes
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All Length Conversions
Other Length Units and Conversions
Explore other length units and their conversion options:
- Meter (m) • Inch to Meter
- Hectometer (hm) • Inch to Hectometer
- Decimeter (dm) • Inch to Decimeter
- Centimeter (cm) • Inch to Centimeter
- Millimeter (mm) • Inch to Millimeter
- Foot (ft) • Inch to Foot
- Yard (yd) • Inch to Yard
- Mile (mi) • Inch to Mile
- Nautical Mile (NM) • Inch to Nautical Mile
- Micrometer (μm) • Inch to Micrometer
Verified Against Authority Standards
All conversion formulas have been verified against international standards and authoritative sources to ensure maximum accuracy and reliability.
National Institute of Standards and Technology — Official US standards for length measurements
Bureau International des Poids et Mesures — International System of Units official documentation
Last verified: December 3, 2025