Imperial & Metric Length
Last reviewed: May 2026
Convert between imperial (inches, feet, yards, miles) and metric (mm, cm, m, km) length units. The fundamental bridge is 1 inch = 2.54 cm exactly — every other conversion derives from this. Construction, sewing, engineering, and international commerce all require frequent length conversions.1
| From | To | Multiply By |
|---|---|---|
| Inches | Centimeters | 2.54 |
| Feet | Meters | 0.3048 |
| Yards | Meters | 0.9144 |
| Miles | Kilometers | 1.60934 |
| Centimeters | Inches | 0.3937 |
| Meters | Feet | 3.28084 |
| Kilometers | Miles | 0.62137 |
Length measurement spans scales from atomic (nanometers) to astronomical (light-years). For everyday purposes, the key systems are Imperial/U.S. customary (inches, feet, yards, miles) and Metric (SI) (millimeters, centimeters, meters, kilometers). The fundamental conversion: 1 inch = 2.54 centimeters (exact, by definition since 1959). From this single relationship, all other length conversions derive.
1 meter = 3.281 feet = 39.37 inches. 1 foot = 30.48 cm = 0.3048 m. 1 mile = 1.609 km = 5,280 feet. 1 kilometer = 0.621 miles = 3,281 feet. 1 yard = 0.9144 m = 3 feet. Quick mental conversion: multiply km by 0.6 for approximate miles (100 km ≈ 60 miles). For meters to feet: multiply by 3.3 (10 m ≈ 33 feet).
The precision needed depends on context. Construction: Framing tolerances are ±⅛" — a 16' wall measured at 16'¼" creates compounding problems. Machining: Precision metalwork operates in thousandths of an inch (mils) or hundredths of a millimeter. Surveying: Land measurements use tenths and hundredths of a foot, not inches. Fabric and sewing: Measured in inches (U.S.) or centimeters (everywhere else) — a European pattern in centimeters must be converted before cutting American fabric.
Nautical miles (1.852 km = 1.151 statute miles) are used in aviation and maritime navigation because they correspond to one minute of latitude, making chart reading intuitive. A knot is one nautical mile per hour. Land distances on road signs use kilometers in metric countries and miles in the U.S., UK, and a handful of others. The distinction matters for fuel planning: a car getting 30 mpg needs different fuel stops than one rated at 8 L/100km — these measure efficiency in opposite directions (higher mpg = better, lower L/100km = better), and direct conversion requires the formula: 235.215 ÷ L/100km = mpg.
Measurement conversion errors have caused notable disasters beyond the Mars Orbiter. The Gimli Glider (1983) ran out of fuel mid-flight because ground crew calculated fuel in pounds when the aircraft's systems used kilograms. A Korean Air cargo flight in 1999 crashed partly due to confusion between meters and feet in altitude readouts. The Vasa warship (1628) sank on its maiden voyage partly because two construction teams used different length standards (Swedish feet vs Amsterdam feet). These examples illustrate why unambiguous units and double-checked conversions are critical in engineering, aviation, and construction.
Tape measures: Standard hardware store tape measures are accurate to ±1/16" over 12 feet. Higher-quality tapes (Stanley FatMax, Milwaukee) maintain accuracy over longer distances. The hook at the end is intentionally loose — it slides by exactly its own thickness to give accurate measurements whether hooked over an edge or pushed against a surface. Rulers and yardsticks: Accurate for short measurements but prone to parallax error — always read directly above the mark, not at an angle. Laser measures: Accurate to ±1/16" at distances up to 100+ feet. Invaluable for room dimensions, ceiling heights, and outdoor measurements. Wheel measures: Used for road distances and large outdoor areas, accurate to ±1% over long distances.
U.S. construction uses a unique fractional system: dimensions are expressed in feet, inches, and fractions (16ths being the finest common division). A measurement of 8' 4-3/16" means 8 feet, 4 and three-sixteenths inches. Lumber dimensions are nominal, not actual: a "2×4" actually measures 1.5" × 3.5", a "2×6" is 1.5" × 5.5". This discrepancy dates to when lumber was sold at rough-sawn dimensions before planing. Sheet goods (plywood, drywall) are true dimensions: a 4'×8' sheet measures 48" × 96". Understanding these conventions prevents costly cutting errors.
Beyond everyday scales, specialized units handle extreme distances. 1 astronomical unit (AU) = 93 million miles (Earth-Sun distance). 1 light-year = 5.88 trillion miles = 9.46 trillion km. 1 parsec = 3.26 light-years = 19.2 trillion miles. The nearest star (Proxima Centauri) is 4.24 light-years away. The Milky Way galaxy is about 100,000 light-years across. At the other extreme, atomic distances use nanometers (1 nm = 0.000000001 m) and angstroms (1 Å = 0.1 nm). A human hair is about 70,000 nm (70 micrometers) in diameter.
International clothing sizes require length conversions: U.S. men's inseam is measured in inches (30", 32", 34"), while European sizes use centimeters. A 32" inseam = 81 cm. Shoe sizes use completely different scales: U.S. men's 10 = EU 43 = UK 9 = 28 cm foot length. Height conversions are common in international contexts: 5'10" = 177.8 cm = 1.78 m. Medical records worldwide use centimeters for height and metric for growth charts. When traveling for medical care or filling out international forms, having these conversions ready prevents errors in dosing, equipment sizing, and identification.
Land area uses its own measurement ecosystem. 1 acre = 43,560 sq ft = 4,047 sq m ≈ 0.405 hectares. A hectare = 10,000 sq m = 2.471 acres. An acre is roughly the size of a football field (minus the end zones). In U.S. real estate, lot sizes are usually quoted in square feet or acres. In most of the world, hectares or square meters are standard. Farm acreage in the U.S. averages 463 acres per farm; in Europe, the average is about 40 hectares (99 acres). Understanding these conversions is essential when comparing international property listings or agricultural statistics.
The level of precision required varies dramatically by application, and choosing the wrong measurement tool wastes time or causes errors. A tape measure accurate to 1/16 inch (1.6 mm) is more than sufficient for framing a wall, but woodworking joints require 1/32 inch (0.8 mm) precision achievable with a combination square and marking knife. Machining tolerances go further: CNC mills routinely hold 0.001 inch (0.025 mm), and precision grinding achieves 0.0001 inch (0.0025 mm). At the extreme, semiconductor manufacturing measures features in nanometers — a 5 nm transistor gate is about 20 atoms wide. For everyday projects, the important principle is matching precision to purpose: measuring curtain fabric to the nearest millimeter is wasted effort, while cutting granite countertops to the nearest millimeter is essential for a proper fit. Laser distance measurers have largely replaced tape measures for room dimensions, offering accuracy to 1/16 inch over distances up to 300 feet without needing a second person to hold the end.
Property boundaries use length units that rarely appear elsewhere. In the US public land survey system, a chain equals 66 feet (20.117 m), and 80 chains make one mile. An acre is 10 square chains — which is why it equals 43,560 square feet (a number that seems arbitrary until you know it's 66 × 660). Property deeds in older states often reference rods (16.5 feet), links (7.92 inches — one hundredth of a chain), and perches (same as a rod). Modern surveys use feet and decimal feet rather than feet-and-inches: a surveyor marks a boundary at 127.45 feet, not 127 feet 5-3/8 inches, because decimal math eliminates fraction errors in area calculations. GPS-based surveying now achieves centimeter-level accuracy using Real-Time Kinematic (RTK) correction, but legal boundary disputes still reference the original survey monuments and their historically measured distances, which may differ from modern measurements by several feet due to the crude instruments available in the 18th and 19th centuries.
Beyond terrestrial measurement, length units scale to match the vastness of space. An astronomical unit (AU) — the average Earth-Sun distance of 149.6 million km (93 million miles) — works for describing the solar system: Mars orbits at 1.52 AU, Jupiter at 5.20 AU. A light-year (9.461 trillion km) measures interstellar distances: the nearest star system, Alpha Centauri, is 4.37 light-years away. For galactic and intergalactic distances, astronomers use the parsec (3.262 light-years, 30.86 trillion km), defined as the distance at which one astronomical unit subtends one arcsecond of parallax — a definition rooted in the actual measurement technique of stellar parallax. The observable universe spans about 93 billion light-years in diameter, a number so large that even light-years become unwieldy and astronomers express it as roughly 28.5 gigaparsecs. Converting between these scales requires powers of ten: 1 parsec = 3.086 × 10¹³ kilometers = 1.917 × 10¹³ miles.
The level of precision required varies dramatically by application, and choosing the wrong measurement tool wastes time or causes errors. A tape measure accurate to 1/16 inch (1.6 mm) is more than sufficient for framing a wall, but woodworking joints require 1/32 inch (0.8 mm) precision achievable with a combination square and marking knife. Machining tolerances go further: CNC mills routinely hold 0.001 inch (0.025 mm), and precision grinding achieves 0.0001 inch (0.0025 mm). At the extreme, semiconductor manufacturing measures features in nanometers — a 5 nm transistor gate is about 20 atoms wide. For everyday projects, the important principle is matching precision to purpose: measuring curtain fabric to the nearest millimeter is wasted effort, while cutting granite countertops to the nearest millimeter is essential for a proper fit. Laser distance measurers have largely replaced tape measures for room dimensions, offering accuracy to 1/16 inch over distances up to 300 feet without needing a second person to hold the end.
Property boundaries use length units that rarely appear elsewhere. In the US public land survey system, a chain equals 66 feet (20.117 m), and 80 chains make one mile. An acre is 10 square chains — which is why it equals 43,560 square feet (a number that seems arbitrary until you know it's 66 × 660). Property deeds in older states often reference rods (16.5 feet), links (7.92 inches — one hundredth of a chain), and perches (same as a rod). Modern surveys use feet and decimal feet rather than feet-and-inches: a surveyor marks a boundary at 127.45 feet, not 127 feet 5-3/8 inches, because decimal math eliminates fraction errors in area calculations. GPS-based surveying now achieves centimeter-level accuracy using Real-Time Kinematic (RTK) correction, but legal boundary disputes still reference the original survey monuments and their historically measured distances, which may differ from modern measurements by several feet due to the crude instruments available in the 18th and 19th centuries.
Beyond terrestrial measurement, length units scale to match the vastness of space. An astronomical unit (AU) — the average Earth-Sun distance of 149.6 million km (93 million miles) — works for describing the solar system: Mars orbits at 1.52 AU, Jupiter at 5.20 AU. A light-year (9.461 trillion km) measures interstellar distances: the nearest star system, Alpha Centauri, is 4.37 light-years away. For galactic and intergalactic distances, astronomers use the parsec (3.262 light-years, 30.86 trillion km), defined as the distance at which one astronomical unit subtends one arcsecond of parallax — a definition rooted in the actual measurement technique of stellar parallax. The observable universe spans about 93 billion light-years in diameter, a number so large that even light-years become unwieldy and astronomers express it as roughly 28.5 gigaparsecs. Converting between these scales requires powers of ten: 1 parsec = 3.086 × 10¹³ kilometers = 1.917 × 10¹³ miles.
The level of precision required varies dramatically by application, and choosing the wrong measurement tool wastes time or causes errors. A tape measure accurate to 1/16 inch (1.6 mm) is more than sufficient for framing a wall, but woodworking joints require 1/32 inch (0.8 mm) precision achievable with a combination square and marking knife. Machining tolerances go further: CNC mills routinely hold 0.001 inch (0.025 mm), and precision grinding achieves 0.0001 inch (0.0025 mm). At the extreme, semiconductor manufacturing measures features in nanometers — a 5 nm transistor gate is about 20 atoms wide. For everyday projects, the important principle is matching precision to purpose: measuring curtain fabric to the nearest millimeter is wasted effort, while cutting granite countertops to the nearest millimeter is essential for a proper fit. Laser distance measurers have largely replaced tape measures for room dimensions, offering accuracy to 1/16 inch over distances up to 300 feet without needing a second person to hold the end.
Property boundaries use length units that rarely appear elsewhere. In the US public land survey system, a chain equals 66 feet (20.117 m), and 80 chains make one mile. An acre is 10 square chains — which is why it equals 43,560 square feet (a number that seems arbitrary until you know it's 66 × 660). Property deeds in older states often reference rods (16.5 feet), links (7.92 inches — one hundredth of a chain), and perches (same as a rod). Modern surveys use feet and decimal feet rather than feet-and-inches: a surveyor marks a boundary at 127.45 feet, not 127 feet 5-3/8 inches, because decimal math eliminates fraction errors in area calculations. GPS-based surveying now achieves centimeter-level accuracy using Real-Time Kinematic (RTK) correction, but legal boundary disputes still reference the original survey monuments and their historically measured distances, which may differ from modern measurements by several feet due to the crude instruments available in the 18th and 19th centuries.
Beyond terrestrial measurement, length units scale to match the vastness of space. An astronomical unit (AU) — the average Earth-Sun distance of 149.6 million km (93 million miles) — works for describing the solar system: Mars orbits at 1.52 AU, Jupiter at 5.20 AU. A light-year (9.461 trillion km) measures interstellar distances: the nearest star system, Alpha Centauri, is 4.37 light-years away. For galactic and intergalactic distances, astronomers use the parsec (3.262 light-years, 30.86 trillion km), defined as the distance at which one astronomical unit subtends one arcsecond of parallax — a definition rooted in the actual measurement technique of stellar parallax. The observable universe spans about 93 billion light-years in diameter, a number so large that even light-years become unwieldy and astronomers express it as roughly 28.5 gigaparsecs. Converting between these scales requires powers of ten: 1 parsec = 3.086 × 10¹³ kilometers = 1.917 × 10¹³ miles.
The level of precision required varies dramatically by application, and choosing the wrong measurement tool wastes time or causes errors. A tape measure accurate to 1/16 inch (1.6 mm) is more than sufficient for framing a wall, but woodworking joints require 1/32 inch (0.8 mm) precision achievable with a combination square and marking knife. Machining tolerances go further: CNC mills routinely hold 0.001 inch (0.025 mm), and precision grinding achieves 0.0001 inch (0.0025 mm). At the extreme, semiconductor manufacturing measures features in nanometers — a 5 nm transistor gate is about 20 atoms wide. For everyday projects, the important principle is matching precision to purpose: measuring curtain fabric to the nearest millimeter is wasted effort, while cutting granite countertops to the nearest millimeter is essential for a proper fit. Laser distance measurers have largely replaced tape measures for room dimensions, offering accuracy to 1/16 inch over distances up to 300 feet without needing a second person to hold the end.
Property boundaries use length units that rarely appear elsewhere. In the US public land survey system, a chain equals 66 feet (20.117 m), and 80 chains make one mile. An acre is 10 square chains — which is why it equals 43,560 square feet (a number that seems arbitrary until you know it's 66 × 660). Property deeds in older states often reference rods (16.5 feet), links (7.92 inches — one hundredth of a chain), and perches (same as a rod). Modern surveys use feet and decimal feet rather than feet-and-inches: a surveyor marks a boundary at 127.45 feet, not 127 feet 5-3/8 inches, because decimal math eliminates fraction errors in area calculations. GPS-based surveying now achieves centimeter-level accuracy using Real-Time Kinematic (RTK) correction, but legal boundary disputes still reference the original survey monuments and their historically measured distances, which may differ from modern measurements by several feet due to the crude instruments available in the 18th and 19th centuries.
Beyond terrestrial measurement, length units scale to match the vastness of space. An astronomical unit (AU) — the average Earth-Sun distance of 149.6 million km (93 million miles) — works for describing the solar system: Mars orbits at 1.52 AU, Jupiter at 5.20 AU. A light-year (9.461 trillion km) measures interstellar distances: the nearest star system, Alpha Centauri, is 4.37 light-years away. For galactic and intergalactic distances, astronomers use the parsec (3.262 light-years, 30.86 trillion km), defined as the distance at which one astronomical unit subtends one arcsecond of parallax — a definition rooted in the actual measurement technique of stellar parallax. The observable universe spans about 93 billion light-years in diameter, a number so large that even light-years become unwieldy and astronomers express it as roughly 28.5 gigaparsecs. Converting between these scales requires powers of ten: 1 parsec = 3.086 × 10¹³ kilometers = 1.917 × 10¹³ miles.
The level of precision required varies dramatically by application, and choosing the wrong measurement tool wastes time or causes errors. A tape measure accurate to 1/16 inch (1.6 mm) is more than sufficient for framing a wall, but woodworking joints require 1/32 inch (0.8 mm) precision achievable with a combination square and marking knife. Machining tolerances go further: CNC mills routinely hold 0.001 inch (0.025 mm), and precision grinding achieves 0.0001 inch (0.0025 mm). At the extreme, semiconductor manufacturing measures features in nanometers — a 5 nm transistor gate is about 20 atoms wide. For everyday projects, the important principle is matching precision to purpose: measuring curtain fabric to the nearest millimeter is wasted effort, while cutting granite countertops to the nearest millimeter is essential for a proper fit. Laser distance measurers have largely replaced tape measures for room dimensions, offering accuracy to 1/16 inch over distances up to 300 feet without needing a second person to hold the end.
Property boundaries use length units that rarely appear elsewhere. In the US public land survey system, a chain equals 66 feet (20.117 m), and 80 chains make one mile. An acre is 10 square chains — which is why it equals 43,560 square feet (a number that seems arbitrary until you know it's 66 × 660). Property deeds in older states often reference rods (16.5 feet), links (7.92 inches — one hundredth of a chain), and perches (same as a rod). Modern surveys use feet and decimal feet rather than feet-and-inches: a surveyor marks a boundary at 127.45 feet, not 127 feet 5-3/8 inches, because decimal math eliminates fraction errors in area calculations. GPS-based surveying now achieves centimeter-level accuracy using Real-Time Kinematic (RTK) correction, but legal boundary disputes still reference the original survey monuments and their historically measured distances, which may differ from modern measurements by several feet due to the crude instruments available in the 18th and 19th centuries.
Beyond terrestrial measurement, length units scale to match the vastness of space. An astronomical unit (AU) — the average Earth-Sun distance of 149.6 million km (93 million miles) — works for describing the solar system: Mars orbits at 1.52 AU, Jupiter at 5.20 AU. A light-year (9.461 trillion km) measures interstellar distances: the nearest star system, Alpha Centauri, is 4.37 light-years away. For galactic and intergalactic distances, astronomers use the parsec (3.262 light-years, 30.86 trillion km), defined as the distance at which one astronomical unit subtends one arcsecond of parallax — a definition rooted in the actual measurement technique of stellar parallax. The observable universe spans about 93 billion light-years in diameter, a number so large that even light-years become unwieldy and astronomers express it as roughly 28.5 gigaparsecs. Converting between these scales requires powers of ten: 1 parsec = 3.086 × 10¹³ kilometers = 1.917 × 10¹³ miles.
→ Remember: 1 in = 2.54 cm. Everything else derives from this.
→ Quick mile→km: Multiply by 1.6.
→ For construction: Use our lumber and concrete calculators for material dimensions.
→ Metric prefixes: milli (1/1000), centi (1/100), kilo (1000).
See also: Temperature · Weight · Volume · Speed