Why an Inch Is Exactly 25.4 Millimetres
Every imperial length unit is now defined from the metre (meter in US spelling). Under the International Yard and Pound Agreement, which took effect on 1 July 1959, the inch became exactly 25.4 millimetres. Before that, the United States defined its inch from the relationship of 39.37 inches to a metre, which makes it about 25.40005 mm, and the British inch was slightly shorter, so the two differed by around two parts per million. That is invisible on a ruler but not on a precision-machined part, so the countries agreed one value.
The foot, yard and mile all follow from that single number. A foot is exactly 0.3048 m, so a 40-foot shipping container is exactly 12.192 m long (see the feet to metres converter), and a 20 m cable is about 65.62 ft (see the metres to feet converter).
| Unit | Definition | In metres |
|---|---|---|
| Inch | 25.4 mm | 0.0254 m |
| Foot | 12 inches | 0.3048 m |
| Yard | 3 feet | 0.9144 m |
| Statute mile | 5,280 feet | 1,609.344 m |
| Nautical mile | Defined directly | 1,852 m |
| US survey foot (retired) | 1,200 ÷ 3,937 m | 0.3048006 m |
The last row shows that the story had a loose end. The United States kept a separate "survey foot", about two parts per million longer than the international foot, so that old land records would not shift. NIST and the National Geodetic Survey retired it at the end of 2022, so the international foot is now the only one in use.
The metre itself is defined from the speed of light: it is the distance light travels in a vacuum in 1/299,792,458 of a second, which fixes the speed of light at exactly 299,792,458 m/s. Every unit in the table above is therefore a fixed multiple of a distance that can be reproduced anywhere.
Length Units from Atoms to Stars
Length units span more than forty orders of magnitude, from atomic dimensions to interstellar distances. Different fields use different units because each unit gives numbers that are easy to work with at its own scale.
| Unit | Size in metres | Typical use |
|---|---|---|
| Nanometre (nm) | 10−9 m | Wavelengths of light, molecules, semiconductor features |
| Micrometre (µm) | 10−6 m | Cells, fibres, coatings and manufacturing tolerances. The older name is micron |
| Millimetre (mm) | 0.001 m | Small product dimensions, fasteners, rainfall, engineering drawings |
| Centimetre (cm) | 0.01 m | Everyday objects, clothing and body dimensions |
| Metre (m) | 1 m | The SI base unit: rooms, buildings, athletics, engineering |
| Kilometre (km) | 1,000 m | Road distances and geography |
| Ångström (Å) | 10−10 m | Atoms, chemical bonds and crystallography (0.1 nm) |
| Inch (in) | 0.0254 m | Screens, pipes, tools and construction materials |
| Foot (ft) | 0.3048 m | Height, rooms, construction and elevation in the United States |
| Yard (yd) | 0.9144 m | Fabric, sports fields and landscaping |
| Statute mile (mi) | 1,609.344 m | Road distances in the United States and United Kingdom |
| Nautical mile | 1,852 m | Maritime and aviation navigation, because it is tied to latitude |
| Mil | 0.0000254 m | Thickness of wire, coatings and sheet material (one-thousandth of an inch). Not the same as the angular mil |
| Rod | 5.0292 m | Historical land measurement (16.5 feet), still in older property records |
| Chain | 20.1168 m | Traditional surveying unit (66 feet, or four rods). Ten square chains make one acre |
| Furlong | 201.168 m | One-eighth of a mile (660 feet), familiar from horse racing |
| Astronomical unit (au) | 149,597,870,700 m (exact) | Distances within planetary systems, close to the average Earth to Sun distance |
| Light-year (ly) | about 9.4607 × 1015 m | Distance light travels in a vacuum in one Julian year. A unit of distance, not time |
| Parsec (pc) | about 3.0857 × 1016 m | Distances to stars and galaxies, about 3.26 light-years |
The metre and its multiples follow the SI Brochure of the International Bureau of Weights and Measures, which also defines the astronomical unit and the nautical mile. US customary, historical and astronomical conversion factors are listed in the NIST Guide to the SI.