| Quantity | SI | US customary | Imperial | CGS |
|---|---|---|---|---|
| Length | metre (m) | foot (ft) = 0.3048 m | yard (yd) = 0.9144 m | centimetre (cm) = 0.01 m |
| Mass | kilogram (kg) | pound (lb) = 0.45359237 kg | pound (lb) = 0.45359237 kg | gram (g) = 0.001 kg |
| Time | second (s) | second (s) | second (s) | second (s) |
| Volume | cubic metre (m³); litre = 0.001 m³ | US gallon = 3.785411784 L | imperial gallon = 4.54609 L | cubic centimetre (cm³) |
| Force | newton (N) | pound-force (lbf) = 4.448221615 N | pound-force (lbf) | dyne = 10⁻⁵ N |
| Energy | joule (J) | British thermal unit = 1,055.055853 J | British thermal unit | erg = 10⁻⁷ J |
| Pressure | pascal (Pa) | psi = 6,894.757293 Pa | psi | barye = 0.1 Pa |
| Temperature | kelvin (K); °C in practice | degree Fahrenheit (°F) | °C officially; °F in everyday use | kelvin / °C |
The International System of Units (SI)
SI is built on seven base units — metre, kilogram, second, ampere, kelvin, mole and candela — defined since 2019 by fixing the numerical values of physical constants. Everything else, from the newton to the tesla, is derived from those seven, and decimal prefixes (kilo-, milli-, micro-) scale any unit by powers of ten. It is the legal system of measurement in almost every country.
US customary units
The US system descends from English units of the colonial era. Since 1959 its length and mass units are defined exactly in SI terms (1 yd = 0.9144 m, 1 lb = 0.45359237 kg). Its liquid gallon is the old English wine gallon of 231 cubic inches, which is why it is smaller than the imperial gallon. US dry measures (bushel, peck) are separate again.
British imperial units
The imperial system was standardised by the Weights and Measures Act of 1824. Imperial lengths and masses match US customary ones, but volumes do not: the imperial gallon is defined as exactly 4.54609 litres, and the imperial pint has 20 fluid ounces rather than 16. The UK has used metric units officially for decades, but miles, pints and stones remain in daily use.
CGS (centimetre–gram–second)
CGS was the dominant scientific system before SI. It survives in astronomy, some physics texts and in magnetism (gauss, oersted, maxwell). Electromagnetic CGS units come in several variants (ESU, EMU, Gaussian) whose equations differ from SI by factors of 4π and the speed of light.
Same name, different size
| US gallons (gal) | Imperial gallons (gal (UK)) |
|---|---|
| 1 gal | 0.83267418 gal (UK) |
| 5 gal | 4.1633709 gal (UK) |
| 10 gal | 8.3267418 gal (UK) |
| 20 gal | 16.653484 gal (UK) |
Other traps: the US ton (2,000 lb) vs the imperial long ton (2,240 lb) vs the metric tonne (1,000 kg); US vs imperial fluid ounces; and the US survey foot, which differs from the international foot by about 2 parts per million.