The SI sits at the bottom
Since the 2019 revision of the International System of Units, the metre, kilogram and kelvin are all fixed by constants of nature: the speed of light, the Planck constant and the Boltzmann constant. The BIPM’s SI Brochure (9th edition) is the reference. Litres, hectares and tonnes are metric units defined directly from these.
1959: the yard and pound get metric definitions
Before 1959, the American and British inches differed slightly. The international yard and pound agreement set the yard at exactly 0.9144 m and the pound at exactly 0.453 592 37 kg. Everything else follows by counting:
- inch = 1/36 yard = 2.54 cm
- foot = 12 inches = 0.3048 m
- mile = 5280 feet = 1609.344 m
- ounce = 1/16 pound = 28.349 523 125 g
- stone = 14 pounds = 6.350 293 18 kg
NIST Special Publication 811 lists these factors in Appendix B. In 2022 the US retired its separate “survey foot”, so there is now one foot.
US and imperial volume split long ago
The US gallon descends from the old English wine gallon, fixed at 231 cubic inches. Britain replaced its gallons in 1824 with the imperial gallon, now defined as exactly 4.546 09 litres. The fluid ounce, pint and quart differ in each system as a result. A US cup is 8 US fluid ounces (236.588 236 5 mL); the US nutrition label uses a rounded 240 mL cup under FDA rules; and Australia, Canada and New Zealand use a 250 mL metric cup.
Temperature needs an offset
Celsius is the kelvin shifted by 273.15. Fahrenheit uses degrees 5/9 the size of a Celsius degree, with water freezing at 32 °F. That offset is why temperature cannot be converted with a single multiplier and why a table like “double it and add 30” drifts off away from 10 °C.
Nautical units come from the globe
The nautical mile was chosen to match one minute of latitude, and has been fixed at exactly 1852 m since the International Hydrographic Conference of 1929. A knot is one nautical mile per hour. Sailors and pilots keep it because it maps straight onto charts.
Before satellite positioning, navigators even timed the Moon against the stars to work out longitude. For a gentler lunar question, ahaboo has a narrated explainer on why the Moon has phases.
Sources
- BIPM, The International System of Units (SI Brochure), 9th edition, 2019.
- NIST Special Publication 811, Guide for the Use of the International System of Units, 2008, Appendix B.
- NIST Handbook 44, Appendix C, General Tables of Units of Measurement.
- UK Weights and Measures Act 1985, Schedule 1.
- US FDA, 21 CFR 101.9(b)(5)(viii) (household measure, cup = 240 mL).
- Federal Register 85 FR 62698 (2020), deprecation of the US survey foot.
Questions
Why is an inch exactly 2.54 cm?
In 1959 the US, UK, Canada, Australia, New Zealand and South Africa agreed on a common yard of 0.9144 m. An inch is 1/36 of that yard, which is exactly 2.54 cm.
Is a US gallon the same as a UK gallon?
No. The US gallon is 231 cubic inches (3.785 411 784 L). The imperial gallon, defined in the UK Weights and Measures Act 1985, is 4.546 09 L.
What defines the kilogram now?
Since 20 May 2019 the kilogram is defined by fixing the numerical value of the Planck constant, replacing the platinum-iridium prototype kept near Paris.