Measure · 33 units
Length
Since 1983 the metre has been defined by the speed of light, and since 1959 the inch, foot, yard and mile have been defined as exact fractions of the metre. Every length unit below is therefore exact unless it is marked otherwise.
Convert length
Then and now
Before the 1959 agreement the American and British inches differed slightly, and the US kept a separate survey foot (1200/3937 m) for land measurement until 2023. Nautical miles also differed: Britain used 6080 feet until 1970.
All length units
“Exact” means the unit is defined as an exact multiple of the SI unit; the decimal is printed in full when it ends. Rows marked historical or obsolete are kept for reading old documents.
| Unit | Defined as | In SI | Status | Source |
|---|---|---|---|---|
| SI | ||||
| micrometre µm | 10⁻⁶ m (once called the micron) | 0.000 001 m | exact | BIPM SI Brochure |
| millimetre mm | 1/1000 m | 0.001 m | exact | BIPM SI Brochure |
| centimetre cm | 1/100 m | 0.01 m | exact | BIPM SI Brochure |
| metre m | SI base unit: the length light travels in vacuum in 1/299 792 458 s | 1 m | exact | BIPM SI Brochure |
| kilometre km | 1000 m | 1 000 m | exact | BIPM SI Brochure |
| Imperial / US | ||||
| thou (mil) mil | 1/1000 inch | 0.000 025 4 m | exact | NIST Handbook 44 |
| line | 1/12 inch | 0.002 116 666 666 67… m | exact historical | NIST Handbook 44 |
| inch in | exactly 25.4 mm | 0.025 4 m | exact Before 1959 the US inch was 1/39.37 m and the British inch differed in the sixth figure. | Yard and pound, 1959 |
| foot ft | 12 inches = 0.3048 m | 0.304 8 m | exact | Yard and pound, 1959 |
| yard yd | 3 feet = 0.9144 m | 0.914 4 m | exact | Yard and pound, 1959 |
| mile (statute) mi | 5280 feet = 1609.344 m | 1 609.344 m | exact | Yard and pound, 1959 |
| Nautical | ||||
| fathom ftm | 6 feet = 1.8288 m | 1.828 8 m | exact | NIST Handbook 44 |
| shackle (of anchor cable) | 15 fathoms = 90 feet | 27.432 m | exact | NIST Handbook 44 |
| cable (international) | 1/10 international nautical mile = 185.2 m | 185.2 m | exact The British cable was 608 feet (1/10 of the old Admiralty mile); the US Navy cable was 120 fathoms. | IHO 1929 / BIPM |
| nautical mile NM | exactly 1852 m | 1 852 m | exact Chosen as close to one minute of latitude, which varies from about 1843 m at the equator to 1862 m at the poles. | IHO 1929 / BIPM |
| Admiralty nautical mile | 6080 feet = 1853.184 m | 1 853.184 m | exact obsolete Used in Britain until 1970. | NIST Handbook 44 |
| US nautical mile (before 1954) | 1853.248 m | 1 853.248 m | exact obsolete | NIST Handbook 44 |
| geographical mile | one minute of longitude on the equator of the WGS 84 ellipsoid, about 1855.32 m | 1 855.324 846 55… m | approx. historical | WGS 84 |
| league (nautical) | 3 nautical miles | 5 556 m | exact historical | NIST Handbook 44 |
| Surveying | ||||
| link (Gunter’s) | 7.92 inches = 1/100 chain | 0.201 168 m | exact historical | NIST Handbook 44 |
| rod (pole, perch) | 16½ feet | 5.029 2 m | exact historical | NIST Handbook 44 |
| chain (Gunter’s) | 66 feet = 4 rods = 100 links | 20.116 8 m | exact historical | NIST Handbook 44 |
| furlong | 10 chains = 220 yards | 201.168 m | exact Still used in horse racing. | NIST Handbook 44 |
| US surveying | ||||
| US survey foot | 1200/3937 m | 0.304 800 609 601… m | exact obsolete Withdrawn on 1 January 2023; it was 2 parts in a million longer than the international foot. | NIST: US survey foot |
| Old English | ||||
| nail (cloth) | 2¼ inches = 1/16 yard | 0.057 15 m | exact historical | NIST Handbook 44 |
| palm | 3 inches | 0.076 2 m | exact historical | NIST Handbook 44 |
| hand | 4 inches (still used for the height of horses) | 0.101 6 m | exact | NIST Handbook 44 |
| span | 9 inches | 0.228 6 m | exact historical | NIST Handbook 44 |
| cubit | 18 inches (traditional English value) | 0.457 2 m | approx. historical The cubit varied widely between peoples and periods. | NIST Handbook 44 |
| pace (military) | 2½ feet | 0.762 m | approx. historical | NIST Handbook 44 |
| league (statute) | 3 statute miles | 4 828.032 m | exact historical | NIST Handbook 44 |
| Typography | ||||
| point (desktop publishing) pt | 1/72 inch | 0.000 352 777 777 778… m | exact Older printers’ points were slightly smaller (about 1/72.27 inch). | NIST SP 811 |
| Astronomy | ||||
| astronomical unit au | exactly 149 597 870 700 m | 149 597 870 700 m | exact | IAU 2012, Res. B2 |