Parsec to Inch Converter
Convert parsecs to inches with our free online length converter.
Quick Answer
1 Parsec = 1.214843e+18 inches
Formula: Parsec × conversion factor = Inch
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.
Parsec to Inch Calculator
How to Use the Parsec to Inch Calculator:
- Enter the value you want to convert in the 'From' field (Parsec).
- The converted value in Inch 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 Parsec to Inch: Step-by-Step Guide
Converting Parsec to Inch involves multiplying the value by a specific conversion factor, as shown in the formula below.
Formula:
1 Parsec = 1.2148e+18 inchesExample Calculation:
Convert 10 parsecs: 10 × 1.2148e+18 = 1.2148e+19 inches
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.
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Need to convert to other length units?
View all Length conversions →What is a Parsec and a Inch?
and Standards
Geometric Definition
The parsec is defined through trigonometric parallax:
1 parsec = the distance at which 1 astronomical unit (AU) subtends an angle of 1 arcsecond (1″)
Mathematically:
- 1 parsec = 1 AU / tan(1″)
- Since 1″ = 1/3600 degree = π/648,000 radians ≈ 4.8481 × 10⁻⁶ radians
- For small angles: tan(θ) ≈ θ (in radians)
- 1 parsec ≈ 1 AU / 4.8481 × 10⁻⁶ ≈ 206,265 AU
Exact IAU Value
The International Astronomical Union (IAU) defines the parsec exactly as:
1 parsec = 648,000/π AU ≈ 206,264.806247 AU
Using the IAU-defined astronomical unit (1 AU = 149,597,870,700 meters exactly as of 2012):
1 parsec = 30,856,775,814,913,673 meters (exactly)
Or approximately:
- 3.0857 × 10¹⁶ meters
- 30.857 trillion kilometers
- 19.174 trillion miles
Relationship to Light-Year
The light-year (distance light travels in one Julian year) relates to the parsec:
1 parsec ≈ 3.26156 light-years
More precisely: 1 pc = 3.261563777 ly (using Julian year of 365.25 days)
Standard Multiples
Kiloparsec (kpc): 1 kpc = 1,000 pc ≈ 3,262 ly
- Used for distances within galaxies
- Milky Way diameter: ~30 kpc
Megaparsec (Mpc): 1 Mpc = 1,000,000 pc ≈ 3.26 million ly
- Used for intergalactic distances
- Andromeda Galaxy: ~0.77 Mpc
Gigaparsec (Gpc): 1 Gpc = 1,000,000,000 pc ≈ 3.26 billion ly
- Used for cosmological distances
- Observable universe radius: ~14 Gpc
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.
Note: The Parsec is part of the imperial/US customary system, primarily used in the US, UK, and Canada for everyday measurements. The Inch belongs to the imperial/US customary system.
History of the Parsec and Inch
and Evolution
Pre-Parsec Era: The Parallax Quest (1600s-1830s)
The concept of stellar parallax dates to ancient Greek astronomy, but detecting it required centuries of technological advancement.
Galileo Galilei (1610) suggested that if Earth orbits the Sun, nearby stars should show annual parallax shifts against distant background stars. No parallax was detected, leading geocentrists to argue Earth must be stationary.
James Bradley (1728) discovered stellar aberration (apparent star position shifts due to Earth's orbital motion combined with finite light speed), confirming Earth's motion but still failing to detect parallax—stars were simply too distant.
Friedrich Wilhelm Bessel achieved the first successful parallax measurement in 1838 for 61 Cygni, determining a distance of about 10.3 light-years (3.16 parsecs, though the term didn't exist yet). This triumph came using a heliometer—a split-lens telescope enabling precise angular measurements.
Thomas Henderson measured Alpha Centauri's parallax (1832-1833, published 1839), and Friedrich Struve measured Vega's (1837), establishing parallax as the fundamental distance measurement method.
Coining the Term (1913)
Herbert Hall Turner (1861-1930), British astronomer and director of Oxford University Observatory, coined "parsec" in 1913. Before this, astronomers expressed stellar distances awkwardly:
- In astronomical units (requiring numbers in the hundreds of thousands)
- In light-years (popular but not directly tied to measurement method)
- In "parallax seconds" (inverse of parallax angle, but confusing terminology)
Turner recognized that astronomers naturally thought in terms of parallax angles. For a star with parallax angle p (in arcseconds), the distance d is simply:
d (in parsecs) = 1 / p (in arcseconds)
This elegant relationship made the parsec immediately practical. A star with 0.5″ parallax is 2 parsecs away; 0.1″ parallax means 10 parsecs; 0.01″ parallax means 100 parsecs.
IAU Adoption (1922-1938)
The 1922 IAU General Assembly in Rome endorsed the parsec as the standard unit for stellar distances, though adoption wasn't immediate or universal.
The 1938 IAU General Assembly in Stockholm formally standardized the parsec definition based on the astronomical unit and arcsecond, solidifying its status.
By the 1950s, the parsec dominated professional astronomy literature, though popular science continued preferring light-years for general audiences.
Space Age Precision (1960s-Present)
Hipparcos satellite (1989-1993): European Space Agency mission measured parallaxes for 118,000 stars with milliarcsecond precision, extending reliable parsec-based distances to hundreds of parsecs.
Gaia mission (2013-present): ESA's Gaia spacecraft has revolutionized astrometry, measuring parallaxes for 1.8 billion stars with microarcsecond precision. This extends direct parsec measurements to 10,000+ parsecs (10+ kiloparsecs), mapping our galaxy's structure in unprecedented detail.
2012 IAU redefinition: The IAU redefined the astronomical unit as exactly 149,597,870,700 meters (no longer based on Earth's actual orbit, which varies slightly). This made the parsec exactly 648,000/π AU, providing a stable definition independent of Earth's orbital variations.
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.
Common Uses and Applications: parsecs vs inches
Explore the typical applications for both Parsec (imperial/US) and Inch (imperial/US) to understand their common contexts.
Common Uses for parsecs
Stellar Astronomy and Parallax Measurements
The parsec's primary use is measuring stellar distances via trigonometric parallax:
Parallax formula: d (parsecs) = 1 / p (arcseconds)
Ground-based observatories: Measure parallaxes to ~0.01″ accuracy, reliable to ~100 pc
Hipparcos satellite: Measured parallaxes to ~0.001″ (1 milliarcsecond), reliable to ~1,000 pc (1 kpc)
Gaia spacecraft: Measures parallaxes to ~0.00001″ (10 microarcseconds) for bright stars, reliable to ~10 kpc for many stars
Applications:
- Calibrating the cosmic distance ladder (using Cepheid variables, RR Lyrae stars)
- Determining absolute magnitudes of stars
- Studying stellar populations and galactic structure
- Measuring proper motions and space velocities
Galactic Structure and Dynamics
Kiloparsecs (kpc) describe structures within galaxies:
Milky Way structure:
- Galactic center (Sagittarius A*): 8.2 kpc from Sun
- Galactic disk radius: ~15 kpc
- Central bulge: ~1.5 kpc radius
- Spiral arms: trace patterns 10-15 kpc in radius
- Dark matter halo: extends to ~60 kpc
Rotation curves: Plot orbital velocity vs. distance (in kpc) from galactic center, revealing dark matter
Star formation regions: Giant molecular clouds span 10-100 pc
Globular clusters: Orbit 10-60 kpc from galactic center
Extragalactic Astronomy
Megaparsecs (Mpc) measure distances between galaxies:
Galaxy surveys: Map millions of galaxies to distances of 1,000+ Mpc, revealing large-scale structure (walls, filaments, voids)
Tully-Fisher relation: Links galaxy rotation speed to luminosity, enabling distance estimates in Mpc
Type Ia supernovae: Standard candles for measuring distances to 1,000+ Mpc
Galaxy clusters: Typical separation between major clusters ~10-50 Mpc
Superclusters: Structures spanning 100-200 Mpc (like Laniakea Supercluster containing Milky Way)
Cosmology and Universe Expansion
Megaparsecs and gigaparsecs describe cosmological distances:
Hubble constant (H₀): Measured in km/s per Mpc—describes universe expansion rate
- Current value: H₀ ≈ 67-73 (km/s)/Mpc (tension between measurement methods)
- Interpretation: Galaxy 1 Mpc away recedes at ~70 km/s; 100 Mpc away recedes at ~7,000 km/s
Hubble's Law: v = H₀ × d (where d is in Mpc, v is recession velocity)
Comoving distance: Cosmological distance accounting for universe expansion, measured in Mpc or Gpc
Redshift surveys: Map galaxy distribution to 1,000+ Mpc (z ~ 0.1-0.3 redshift)
Baryon acoustic oscillations: ~150 Mpc characteristic scale in galaxy distribution, used as "standard ruler"
Astrophysical Research Papers
Parsecs are the default distance unit in professional astronomy journals:
Observational papers: Report star/galaxy distances in pc, kpc, or Mpc
Theoretical models: Express scale lengths in parsecs (e.g., "disk scale length of 3 kpc")
Computer simulations: Use parsec-based units (or comoving kpc/Mpc for cosmological sims)
Standard convention: Professional astronomers think and calculate in parsecs, converting to light-years only for public communication
When to Use 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.
Additional Unit Information
About Parsec (pc)
What does "parsec" stand for?
Parsec is a portmanteau of "parallax of one arcsecond."
It represents the distance at which Earth's orbital radius (1 AU) subtends an angle of exactly one arcsecond (1/3600 of a degree). British astronomer Herbert Hall Turner coined the term in 1913 to provide a convenient unit directly tied to the parallax measurement method.
How is a parsec measured?
Parsecs are measured using trigonometric parallax:
- Observe a nearby star from Earth when Earth is on one side of its orbit
- Observe the same star six months later when Earth is on the opposite side
- Measure the apparent shift in the star's position against distant background stars
- Half this shift is the parallax angle p (in arcseconds)
- Calculate distance: d = 1/p parsecs
Modern method: Space telescopes like Gaia measure parallax angles with microarcsecond precision, enabling distance measurements to thousands of parsecs.
Is a parsec bigger than a light-year?
Yes, one parsec is significantly larger:
1 parsec ≈ 3.26 light-years
More precisely: 1 pc = 3.261563777 ly
Example: Proxima Centauri at 1.3 parsecs equals 4.24 light-years away.
Why the difference matters: Confusing parsecs with light-years introduces 3× error in distances.
Why do astronomers prefer parsecs over light-years?
Astronomers prefer parsecs for several reasons:
1. Direct observational connection: Parallax angle p (arcseconds) directly gives distance d = 1/p (parsecs). No complicated conversion needed.
2. Professional standard: IAU endorsed parsecs in 1922; they're now universal in research papers and textbooks.
3. Convenient multiples: Kiloparsecs (kpc) for galactic distances, megaparsecs (Mpc) for cosmological distances provide natural scales.
4. Hubble constant units: Universe expansion rate naturally expressed in (km/s)/Mpc.
5. Definition stability: Light-year depends on year length definition (tropical, Julian, sidereal); parsec defined purely by geometry.
Light-years remain popular in public communication because "year" is familiar, while "parallax arcsecond" requires technical knowledge.
How many astronomical units are in a parsec?
1 parsec = 206,265 astronomical units (AU) (approximately)
More precisely: 1 pc = 206,264.806247 AU
This number arises from: 1 pc = 1 AU / tan(1″), and since 1″ = π/648,000 radians:
- 1 pc = 1 AU / (π/648,000) = 648,000/π AU ≈ 206,265 AU
Context: Since 1 AU ≈ 150 million km (Earth-Sun distance), 1 parsec ≈ 31 trillion km.
What is a kiloparsec and megaparsec?
Kiloparsec (kpc): 1 kpc = 1,000 parsecs ≈ 3,262 light-years
- Used for: Galactic-scale distances
- Examples: Sun to Milky Way center (8 kpc), galaxy diameters (10-50 kpc)
Megaparsec (Mpc): 1 Mpc = 1,000,000 parsecs ≈ 3.26 million light-years
- Used for: Intergalactic distances, cosmology
- Examples: Andromeda Galaxy (0.77 Mpc), Virgo Cluster (16.5 Mpc), Hubble constant measured in (km/s)/Mpc
Gigaparsec (Gpc): 1 Gpc = 1,000,000,000 parsecs ≈ 3.26 billion light-years
- Used for: Large-scale cosmological structures
- Example: Observable universe radius (~14 Gpc)
Is the parsec an SI unit?
No, the parsec is not an SI unit. The SI unit of length is the meter (m).
However, the parsec is:
- Recognized by the IAU (International Astronomical Union)
- Accepted for use with SI in astronomy contexts
- Defined exactly in terms of the AU (which is defined exactly in meters)
Why not SI?: The parsec arose naturally from astronomical practice and remains far more practical than expressing stellar distances in meters (which would require numbers like 10¹⁶ to 10²³).
Analogy: Like the electronvolt (eV) in particle physics, the parsec is a specialized unit indispensable to its field despite not being SI.
How far can parallax measure distances?
Ground-based telescopes: ~0.01 arcsecond precision → reliable to ~100 parsecs
Hubble Space Telescope: ~0.001 arcsecond (1 milliarcsecond) → reliable to ~1,000 parsecs (1 kpc)
Hipparcos satellite (1989-1993): ~0.001 arcsecond → 118,000 stars measured to 100-1,000 pc
Gaia spacecraft (2013-present): ~0.00001 arcsecond (10 microarcseconds) for bright stars → reliable to ~10,000 parsecs (10 kpc)
- Measured 1.8 billion stars
- Revolutionary precision enables mapping entire Milky Way disk
Fundamental limit: Stars beyond 10-20 kpc have unmeasurably small parallaxes with current technology. For greater distances, astronomers use indirect methods (Cepheids, Type Ia supernovae, redshift).
Did Han Solo make the Kessel Run in "less than 12 parsecs"?
Famous Star Wars quote: "She made the Kessel Run in less than twelve parsecs."
The issue: Parsec measures distance, not time. Saying "less than 12 parsecs" for a speed achievement is like saying "I drove to work in less than 5 miles."
Fan explanations (retroactive justifications):
- The Kessel Run involves navigating near black holes; a shorter distance means a more dangerous, direct route
- Skilled pilots can shave distance by flying closer to gravitational hazards
- This reinterprets "12 parsecs" as boasting about route optimization, not speed
Real answer: George Lucas likely confused parsecs with a time unit when writing the script. The line became famous enough that later writers invented explanations making it technically correct.
Takeaway: In real astronomy, parsecs always measure distance, never time.
How do parsecs relate to the Hubble constant?
The Hubble constant (H₀) describes universe expansion and is typically expressed as:
H₀ ≈ 70 (km/s)/Mpc
Interpretation: For every megaparsec of distance, recession velocity increases by ~70 km/s.
Examples using Hubble's Law (v = H₀ × d):
- Galaxy 1 Mpc away: recedes at ~70 km/s
- Galaxy 10 Mpc away: recedes at ~700 km/s
- Galaxy 100 Mpc away: recedes at ~7,000 km/s
- Galaxy 1,000 Mpc away: recedes at ~70,000 km/s
Hubble length: c/H₀ ≈ 4,400 Mpc (14.4 billion ly) - characteristic distance scale of observable universe
Why Mpc?: Using megaparsecs keeps Hubble constant values convenient (70 rather than 0.000000000070 if using parsecs, or 2.3 × 10⁻¹⁸ if using SI meters).
What's the farthest distance ever measured in parsecs?
Observable universe radius: ~14,000 Mpc = 14 Gpc (46 billion light-years comoving distance)
Most distant galaxy observed (as of 2023): JADES-GS-z13-0 at redshift z ≈ 13.2
- Comoving distance: ~4,200 Mpc (13.7 billion light-years light-travel distance)
- Due to universe expansion, it's now ~10,000 Mpc (32 billion light-years) away
Cosmic microwave background: Emitted 380,000 years after Big Bang
- Comoving distance to CMB surface: ~14,000 Mpc (46 billion light-years)
Beyond measurement: The observable universe has a finite size (~14 Gpc radius) due to finite age and light speed. Objects beyond this "cosmological horizon" are unobservable because their light hasn't reached us yet.
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.
Is an inch the same at different temperatures?
Technically, no. Metal expands when it gets hot. A steel ruler that is exactly 12 inches at room temperature will be slightly longer in the summer heat. This is why official standards are measured at 20°C (68°F). In high-precision manufacturing, factories are kept at a constant temperature so their measurements don't "drift."
What is the connection between barleycorns and shoe sizes?
The medieval definition of an inch as "three barleycorns" actually survives today in your shoes! One "size" in the UK and US shoe systems is exactly one barleycorn (1/3 of an inch). This is why a Size 10 is 1/3 of an inch longer than a Size 9.
Why is 120 film called 120 if it's 2.4 inches wide?
The name "120" was actually a catalog number given by Kodak in 1901. It doesn't relate to its width in millimeters or inches directly. However, the film is approximately 2.4 inches (61mm) wide, allowing for a variety of image formats like 2.25" x 2.25" (6x6).
What is a "Square Inch" of pressure?
In the US, tire pressure and steam pressure are measured in PSI (Pounds per Square Inch). It tells you how much force is pressing against a one-inch by one-inch square. For example, a car tire typically has 32 PSI.
How many inches are in a "Hands" measurement for horses?
Horses are measured in "hands," and one hand is exactly 4 inches. If a horse is 15 hands high, it means they are 60 inches tall at the shoulder.
What is the "Inch of Mercury" (inHg)?
Meteorologists use "inches of mercury" to measure air pressure. It refers to how high the pressure of the atmosphere can push a column of liquid mercury inside a glass tube. Standard sea-level pressure is about 29.92 inches.
Are "Mils" and "Millimeters" the same?
No! This is a dangerous mistake.
- In the US, a Mil is 1/1000th of an inch (0.001").
- A Millimeter is 1/1000th of a meter. One millimeter is about 39 mils. Mixing them up in manufacturing can lead to catastrophic failures.
Why are TV screens measured diagonally instead of width?
It started as a marketing trick in the early days of TV (when tubes were round). It made the number sound larger! A 55-inch diagonal TV sounds bigger than a 48-inch wide TV, even though they are the same physical object.
Can an inch be divided into 10 parts?
Yes! While tape measures use 8, 16, or 32 parts, engineers and land surveyors often use "Engineers' Scales" where an inch is divided into 10, 20, 30, 40, 50, or 60 parts. This makes it easier to use decimal math (e.g., 0.1", 0.2").
How many inches is a standard "brick"?
A standard US modular brick is 7 5/8 inches long, which allows for a 3/8-inch mortar joint to make the total unit exactly 8 inches long. This "8-inch module" is the secret to why brick patterns fit so perfectly together.
Conversion Table: Parsec to Inch
| Parsec (pc) | Inch (in) |
|---|---|
| 0.5 | 607,421,259,842,519,700 |
| 1 | 1,214,842,519,685,039,400 |
| 1.5 | 1,822,263,779,527,559,200 |
| 2 | 2,429,685,039,370,078,700 |
| 5 | 6,074,212,598,425,197,000 |
| 10 | 12,148,425,196,850,393,000 |
| 25 | 30,371,062,992,125,985,000 |
| 50 | 60,742,125,984,251,970,000 |
| 100 | 121,484,251,968,503,940,000 |
| 250 | 303,710,629,921,259,850,000 |
| 500 | 607,421,259,842,519,700,000 |
| 1,000 | 1,214,842,519,685,039,400,000 |
People Also Ask
How do I convert Parsec to Inch?
To convert Parsec to Inch, enter the value in Parsec 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 Parsec to Inch?
The conversion factor depends on the specific relationship between Parsec and Inch. 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 Inch back to Parsec?
Yes! You can easily convert Inch back to Parsec by using the swap button (⇌) in the calculator above, or by visiting our Inch to Parsec converter page. You can also explore other length conversions on our category page.
Learn more →What are common uses for Parsec and Inch?
Parsec and Inch 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.
Helpful Conversion Guides
Learn more about unit conversion with our comprehensive guides:
📚 How to Convert Units
Step-by-step guide to unit conversion with practical examples.
🔢 Conversion Formulas
Essential formulas for length and other conversions.
⚖️ Metric vs Imperial
Understand the differences between measurement systems.
⚠️ Common Mistakes
Learn about frequent errors and how to avoid them.
All Length Conversions
Other Length Units and Conversions
Explore other length units and their conversion options:
- Meter (m) • Parsec to Meter
- Kilometer (km) • Parsec to Kilometer
- Hectometer (hm) • Parsec to Hectometer
- Decimeter (dm) • Parsec to Decimeter
- Centimeter (cm) • Parsec to Centimeter
- Millimeter (mm) • Parsec to Millimeter
- Foot (ft) • Parsec to Foot
- Yard (yd) • Parsec to Yard
- Mile (mi) • Parsec to Mile
- Nautical Mile (NM) • Parsec to Nautical Mile
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: February 19, 2026