Conversion method
Choose the definition that matches your measurement, then round the result to a useful precision.
in = x × 0.393700787401575
10 cm = 3.937008 in.
| cm | in |
|---|---|
| 1 | 0.393701 |
| 5 | 1.968504 |
| 10 | 3.937008 |
| 25 | 9.84252 |
| 50 | 19.685039 |
| 100 | 39.370079 |
Metric and imperial length
The metric system scales in powers of ten from the meter, which makes conversion within it a matter of moving the decimal point: a kilometer is 1,000 meters, a centimeter is one hundredth, a millimeter one thousandth. Imperial units come from historical body-based and agricultural measures, which is why they use irregular factors — 12 inches to a foot, 3 feet to a yard, 1,760 yards to a mile.
The two systems have been formally linked since 1959, when the international yard and pound agreement fixed the inch at exactly 25.4 millimeters. Every metric-imperial length conversion derives from that single exact definition, so these conversions are exact rather than approximate.
| Unit | In meters | Notes |
|---|---|---|
| Millimeter (mm) | 0.001 m | 1/1000 m |
| Centimeter (cm) | 0.01 m | 1/100 m; 2.54 cm = 1 in exactly |
| Meter (m) | 1 m | SI base unit |
| Kilometer (km) | 1,000 m | 0.621371 mi |
| Inch (in) | 0.0254 m | Exact since 1959 |
| Foot (ft) | 0.3048 m | 12 in |
| Yard (yd) | 0.9144 m | 3 ft |
| Mile (mi) | 1,609.344 m | 5,280 ft; 1.609344 km |
| Nautical mile | 1,852 m | One minute of latitude |
Conversions worth memorizing
A handful of factors cover most everyday needs.
- 1 inch = 2.54 cm exactly
- 1 foot = 30.48 cm exactly
- 1 meter ≈ 3.281 feet or 39.37 inches
- 1 mile = 1.609344 km exactly
- 1 km ≈ 0.621 miles
- 1 yard = 0.9144 m exactly
- 1 nautical mile = 1,852 m exactly (one minute of latitude)
| Value | cm → in | in → cm | m → ft | ft → m | km → mi | mi → km |
|---|---|---|---|---|---|---|
| 1 | 0.394 | 2.54 | 3.281 | 0.305 | 0.621 | 1.609 |
| 5 | 1.969 | 12.7 | 16.404 | 1.524 | 3.107 | 8.047 |
| 10 | 3.937 | 25.4 | 32.808 | 3.048 | 6.214 | 16.093 |
| 25 | 9.843 | 63.5 | 82.021 | 7.62 | 15.534 | 40.234 |
| 50 | 19.685 | 127 | 164.042 | 15.24 | 31.069 | 80.467 |
| 100 | 39.37 | 254 | 328.084 | 30.48 | 62.137 | 160.934 |
Why the nautical mile is different
The nautical mile is not an arbitrary length: it was defined as one minute of arc of latitude along a meridian, which ties it directly to the Earth's geometry. That property makes it uniquely convenient for navigation, since a degree of latitude is 60 nautical miles anywhere on the globe. It is now fixed at exactly 1,852 meters by international agreement, and the knot — one nautical mile per hour — remains the standard speed unit in aviation and at sea.
When conversion errors matter
Unit confusion has caused real and expensive failures. NASA lost the Mars Climate Orbiter in 1999 because one team supplied thrust data in pound-force seconds while the receiving software expected newton-seconds. In 1983, an Air Canada 767 ran out of fuel mid-flight after a loading calculation mixed pounds and kilograms.
The practical lesson is to carry units through a calculation rather than working with bare numbers, and to sanity-check the magnitude of a result. If converting meters to feet produces a smaller number, the factor was applied in the wrong direction.
Height conversion chart
Height is the everyday length conversion most people actually need. Feet and inches to centimetres: multiply the total inches by 2.54. The chart runs from 4′10″ to 6′6″ in two-inch steps.
| Height | Inches | cm | m |
|---|---|---|---|
| 4′10″ | 58 | 147.3 | 1.473 |
| 5′0″ | 60 | 152.4 | 1.524 |
| 5′2″ | 62 | 157.5 | 1.575 |
| 5′4″ | 64 | 162.6 | 1.626 |
| 5′6″ | 66 | 167.6 | 1.676 |
| 5′8″ | 68 | 172.7 | 1.727 |
| 5′10″ | 70 | 177.8 | 1.778 |
| 6′0″ | 72 | 182.9 | 1.829 |
| 6′2″ | 74 | 188 | 1.88 |
| 6′4″ | 76 | 193 | 1.93 |
| 6′6″ | 78 | 198.1 | 1.981 |
Unit definitions and sources
Ordinary feet, yards and miles use international definitions. The separately labeled US survey foot is for legacy records only. A nautical mile is exactly 1,852 meters; it is not a constant minute of latitude on Earth.
19 of the 31 definitions below were checked against the cited sources on 2026-09-20 (AI-assisted check).
| Unit and system | Use | Definition | Relationship |
|---|---|---|---|
| Millimeter (mm); si | Common units | Millimeter measures length. m = (value in mm) × 0.001. Common use: Dimensions and distances. | Exact definition; numerical display may be rounded |
| Centimeter (cm); si | Common units | Centimeter measures length. m = (value in cm) × 0.01. Common use: Dimensions and distances. | Exact definition; numerical display may be rounded |
| Meter (m); si | Common units | Meter measures length. m = (value in m) × 1. Common use: Dimensions and distances. | Exact definition; numerical display may be rounded |
| Kilometer (km); si | Common units | Kilometer measures length. m = (value in km) × 1000. Common use: Dimensions and distances. | Exact definition; numerical display may be rounded |
| Inch (in); us | Common units | Inch measures length. m = (value in in) × 0.0254. Common use: Dimensions and distances. | Exact definition; numerical display may be rounded |
| Foot (ft); us | Common units | Foot measures length. m = (value in ft) × 0.3048. Common use: Dimensions and distances. | Exact definition; numerical display may be rounded |
| Yard (yd); us | Common units | Yard measures length. m = (value in yd) × 0.9144. Common use: Dimensions and distances. | Exact definition; numerical display may be rounded |
| Mile (mi); us | Common units | Mile measures length. m = (value in mi) × 1609.344. Common use: Dimensions and distances. | Exact definition; numerical display may be rounded |
| Nautical mile (nmi); other | Common units | Nautical mile measures length. m = (value in nmi) × 1852. Common use: Dimensions and distances. | Exact definition; numerical display may be rounded |
| Micrometer (µm); si | Specialist units | Micrometer measures length. m = (value in µm) × 0.000001. Common use: Dimensions and distances. | Exact definition; numerical display may be rounded |
| Nanometer (nm); si | Specialist units | Nanometer measures length. m = (value in nm) × 1e-9. Common use: Dimensions and distances. | Exact definition; numerical display may be rounded |
| Light-year (ly); other | Specialist units | Light-year measures length. m = (value in ly) × 9460730472580800. Common use: Dimensions and distances. | Exact definition; numerical display may be rounded |
| Astronomical unit (au); other | Specialist units | An astronomical unit is exactly 149,597,870,700 meters under the IAU 2012 definition. Common use: Solar-system distances. | Exact definition; numerical display may be rounded |
| Angstrom (Å); other | Specialist units | Angstrom: multiply by 1e-10 to obtain the category base unit. Common use: Dimensions and distances. | Exact definition; numerical display may be rounded |
| Mil (0.001 inch) (mil); other | Specialist units | Mil (0.001 inch): multiply by 2.54e-05 to obtain the category base unit. Common use: Dimensions and distances. A length mil is one thousandth of an inch, not an angular mil. | Exact definition; numerical display may be rounded |
| Microinch (µin); other | Specialist units | Microinch: multiply by 2.54e-08 to obtain the category base unit. Common use: Dimensions and distances. | Exact definition; numerical display may be rounded |
| Fathom (international) (fathom); other | Specialist units | Fathom (international): multiply by 1.8288 to obtain the category base unit. Common use: Dimensions and distances. Six international feet. Historical survey fathoms differ. | Exact definition; numerical display may be rounded |
| Chain (international) (ch); other | Specialist units | Chain (international): multiply by 20.1168 to obtain the category base unit. Common use: Dimensions and distances. 66 international feet, not a US survey chain. | Exact definition; numerical display may be rounded |
| Rod (international) (rd); other | Specialist units | Rod (international): multiply by 5.0292 to obtain the category base unit. Common use: Dimensions and distances. 16.5 international feet, not a US survey rod. | Exact definition; numerical display may be rounded |
| Exameter (Em); si | Specialist units | exameter: E denotes 10^18; the unit gives 1000000000000000000 m. Common use: Dimensions and distances. | Exact definition; numerical display may be rounded |
| Petameter (Pm); si | Specialist units | petameter: P denotes 10^15; the unit gives 1000000000000000 m. Common use: Dimensions and distances. | Exact definition; numerical display may be rounded |
| Terameter (Tm); si | Specialist units | terameter: T denotes 10^12; the unit gives 1000000000000 m. Common use: Dimensions and distances. | Exact definition; numerical display may be rounded |
| Gigameter (Gm); si | Specialist units | gigameter: G denotes 10^9; the unit gives 1000000000 m. Common use: Dimensions and distances. | Exact definition; numerical display may be rounded |
| Megameter (Mm); si | Specialist units | megameter: M denotes 10^6; the unit gives 1000000 m. Common use: Dimensions and distances. | Exact definition; numerical display may be rounded |
| Hectometer (hm); si | Specialist units | hectometer: h denotes 10^2; the unit gives 100 m. Common use: Dimensions and distances. | Exact definition; numerical display may be rounded |
| Decameter (dam); si | Specialist units | decameter: da denotes 10^1; the unit gives 10 m. Common use: Dimensions and distances. | Exact definition; numerical display may be rounded |
| Decimeter (dm); si | Specialist units | decimeter: d denotes 10^-1; the unit gives 0.1 m. Common use: Dimensions and distances. | Exact definition; numerical display may be rounded |
| Picometer (pm); si | Specialist units | picometer: p denotes 10^-12; the unit gives 1e-12 m. Common use: Dimensions and distances. | Exact definition; numerical display may be rounded |
| Femtometer (fm); si | Specialist units | femtometer: f denotes 10^-15; the unit gives 1e-15 m. Common use: Dimensions and distances. | Exact definition; numerical display may be rounded |
| Attometer (am); si | Specialist units | attometer: a denotes 10^-18; the unit gives 1e-18 m. Common use: Dimensions and distances. | Exact definition; numerical display may be rounded |
| US survey foot (legacy) (ftUS); us | Legacy units; obsolete | One US survey foot is exactly 1200/3937 meters; it differs from the international foot of exactly 0.3048 meters. Common use: Interpreting legacy US survey records. For historical records only. Retired in 2023; ordinary feet use the international definition. | Exact definition; numerical display may be rounded |
Sources: BIPM: International System of Units; NIST: SP 811 Appendix B.8: conversion factors; IAU: Measuring the Universe: astronomical unit; NIST: US survey foot: definition and retirement; BIPM: SI prefixes
Precision of length conversions
All 31 length units convert through Meter (m). 31 have an exact defined relationship to the base unit and 0 are conventional or measured values; 1 is a legacy definition kept for reading old records. Results and the tables on this page come from the same engine, so a table value can be checked against the converter directly.
Standard display shows up to six decimal places, switching to scientific notation for very large or small values. Calculation uses JavaScript binary floating point independently of display rounding, so extra digits are not a claim about measurement accuracy. Each unit lists its own source; a citation records where the definition comes from, not an independent review. See Conversion methodology for the review and correction process.