This length converter lists 92 units that stretch over about 43.6 orders of magnitude, from the Planck length to the radius of the Sun, and somewhere in the middle it holds two different feet. Those two feet, which differ by two parts per million, deserve a closer look.
A MILE, MEASURED TWO WAYS
By direct calculation with 50-digit arithmetic. A survey foot of exactly 1,200/3,937 m reproduces every US survey value of this converter to the digits shown.
Length Converter
Complete list of length units
| Unit | 1 unit in meters |
|---|
Shaded rows use the updated values of the panel on older tables. A red mark on the left shows units that share one value.
Definition: the Meter, the Foot and 92 Units
The meter is defined as the distance light travels in 1/299,792,458 of a second. The foot has been exactly 0.3048 meters since 1959, which gives an inch of 2.54 centimeters and an international mile of 1,609.344 m. Around those anchors, the converter starts with the meter and the SI prefixes from exameter to attometer, then the mile, yard, foot and inch, and goes on to astronomical, nautical and surveyors' units, printer's units, atomic scales and traditional units named for other cultures, such as the Roman actus or the Russian archin. Seven groups of units share one value, such as micrometer and micron, or rod, perch and pole, which makes 84 distinct values behind 92 names.
History and Origin
The meter was first defined in 1793 as one ten-millionth of the distance from the equator to the North Pole. In 1889 it became a prototype bar of 90% platinum and 10% iridium measured at the melting point of ice, in 1960 a number of wavelengths of a krypton-86 emission line, and since 1983 the distance light travels in 1/299,792,458 of a second, slightly modified in 2019 to follow the redefinition of the second.
The foot has a messier story. Before standardization, feet in the Greek, Roman, English, Chinese and French systems ranged from 250 mm to 335 mm, against 304.8 mm today, because units came from parts of the body.
Current Use, and Two Different Feet
The United States still uses yards, inches, feet and miles in everyday life. A km to miles conversion multiplies by 0.621371192, which is 1,000 ÷ 1,609.344, a meters to feet conversion uses 3.280839895 feet per meter, which is 1 ÷ 0.3048, and a cm to inches conversion divides by 2.54, because the inch is exactly 0.0254 m.
This converter also lists a US survey foot of 0.3048006096 m, together with survey versions of the inch, link, chain, rod, furlong, fathom and mile, and a “league (statute)” that by direct calculation equals three survey miles. By direct calculation the survey foot is 1,200/3,937 m, which is 2.000004 parts per million longer than the international foot: 0.61 micrometers per foot, 3.2 mm per mile, 2 m over 1,000 km. The plain “mile” is the international mile of 1,609.344 m, while “mile (statute)” has exactly the value of “mile (US survey)”.
A notice relayed by the Wisconsin State Cartographer's Office reports that NIST and the National Geodetic Survey deprecated the survey foot as of December 31, 2022. After that date, feet in surveying and mapping should be exactly 0.3048 m, although the survey foot is kept in legacy products such as the State Plane Coordinate Systems of 1983 and 1927.
Where Older Tables Are Out of Date
Eight entries rest on values that have since been superseded, and this converter uses the current ones. Older tables list the astronomical unit at 149,597,870,691 m, but the 2012 resolutions of the International Astronomical Union redefine it as exactly 149,597,870,700 m. A paper on the IAU's 2015 nominal values reports that the parsec was then adopted as exactly 648,000/π astronomical units, so a parsec, kiloparsec or megaparsec computed from the older unit is too short by 6.85 × 10-11 in relative terms. Some tables still carry the values of the 1986 adjustment of the fundamental constants for the Planck length, the Bohr radius (also the atomic unit of length) and the classical electron radius, as reproduced in a copy of that table. The 2022 adjustment, in a copy of the NIST listing and, for the Bohr radius, on Wikipedia's page citing NIST, gives the figures below.
| Unit | Older value | This converter | Reason |
|---|---|---|---|
| astronomical unit | 149,597,870,691 m | 149,597,870,700 m | 2012 definition |
| parsec | 3.08567758128 × 1016 m | 3.0856775814913673 × 1016 m | 648,000/π au |
| kiloparsec | 3.08567758128 × 1019 m | 3.0856775814913673 × 1019 m | 648,000/π au |
| megaparsec | 3.08567758128 × 1022 m | 3.0856775814913673 × 1022 m | 648,000/π au |
| Bohr radius and a.u. of length | 5.29177249 × 10-11 m | 5.29177210544 × 10-11 m | 2022 adjustment |
| classical electron radius | 2.81794092 × 10-15 m | 2.8179403205 × 10-15 m | 2022 adjustment |
| Planck length | 1.61605 × 10-35 m | 1.616255 × 10-35 m | 2022 adjustment |
Frequently Asked Questions
How many feet are in a meter?
3.280839895, which is 1 ÷ 0.3048.
How many centimeters are in an inch?
2.54, because the inch is exactly 0.0254 meters.
How many meters are in a mile?
1,609.344 for the international mile, and 1,609.3472186944 for the US survey mile.
How long is a US survey foot?
0.3048006096 meters, about 0.61 micrometers longer than the international foot.
Why does the converter include the Planck length and the Sun's radius?
Because both are lengths: 1.616255 × 10-35 m for the first, 696,000,000 m for the second, about 43.6 orders of magnitude apart.
Method and sources. Every factor was checked against its exact definition with 50-digit arithmetic. The seven rows of the table above rest on an IAU resolution, a paper quoting one, a copy of a NIST listing and a copy of the 1986 table, and the status of the survey foot on three pages relaying federal notices, not on the primary NIST pages.
This converter is provided for general reference. For surveying, engineering or legal work, use the unit and the standard the document specifies. Written by Staff Editorial, NumeraHub.