Millimeter to Inch Converter

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.