Glossary of unit systems

The International System of Units — the one everything else is defined against.

Since 2019 all seven base units are fixed by exact numerical values of seven defining constants: the caesium hyperfine frequency, the speed of light, the Planck constant, the elementary charge, the Boltzmann constant, the Avogadro constant and the luminous efficacy of 540 THz radiation. No SI unit depends on an artefact any more, which is why so many conversions on this site are exact fractions rather than measurements.

Examples: metre, kilogram, second, ampere, kelvin, mole, candela

Decimal units accepted for use with the SI, or descended from earlier metric systems.

The litre, the tonne, the hectare, the bar and the minute are not SI units, but they are defined as exact multiples of SI units and are used everywhere. A hectare is exactly 10 000 m²; a bar is exactly 100 000 Pa. Nothing is lost converting them.

Examples: litre, tonne, hectare, bar, hour

The British system fixed by the Weights and Measures Act 1824 and its successors.

Imperial and US customary share most length and mass units but differ badly on volume: an imperial gallon is 4.546 09 L, a US liquid gallon is 3.785 411 784 L. An imperial pint is 20 fluid ounces, a US pint is 16. Any recipe or fuel figure that crosses the Atlantic has to say which it means.

Examples: imperial gallon, imperial pint, stone, fluid ounce (imp)

The United States system, defined since 1959 against the metre and the kilogram.

The 1959 international yard and pound agreement made the yard exactly 0.9144 m and the pound exactly 0.453 592 37 kg. That single agreement is why almost every length and mass conversion between US units and SI on this site is exact. The survey foot, kept alive for land measurement until 2023, is the notable exception.

Examples: US gallon, US fluid ounce, short ton, survey foot

Centimetre–gram–second: the system physics used before the SI, still current in parts of it.

CGS units survive where they are convenient: the gauss and the oersted in magnetism, the poise and the stokes in fluid mechanics, the erg and the dyne in older literature. They convert to SI by exact powers of ten, so nothing is approximated — but the electromagnetic CGS variants define their quantities differently, which is a physics problem rather than an arithmetic one.

Examples: dyne, erg, gauss, poise, stokes

Units sized for distances and masses that make metres useless.

The astronomical unit was redefined in 2012 as exactly 149 597 870 700 m, and the Julian light-year follows exactly from the speed of light and a 365.25-day year. The parsec is defined from the astronomical unit and the radian, so it carries π and is therefore irrational — this site says so rather than pretending otherwise.

Examples: astronomical unit, light-year, parsec, solar mass

Units built from physical constants rather than from human-scale artefacts.

Some are exact by definition since 2019 — the electronvolt follows from the elementary charge, which is now fixed. Others, like the unified atomic mass unit and the electron mass, remain measured quantities: their values come from CODATA 2022 and carry an experimental uncertainty, so conversions through them are marked approximate.

Examples: electronvolt, atomic mass unit, bohr radius, hartree

Units no longer in official use, kept because old documents still use them.

A drawing from 1950 or a deed from 1890 does not become unreadable because a unit was retired. These are included with their defining values so historical figures can be brought forward, not because anyone should measure in them today.

Examples: furlong, chain, rod, hogshead, apothecary units

Everything that does not belong to a system: logarithmic, dimensionless and trade units.

Decibels, nepers, pH and musical cents are logarithmic — they have no conversion factor at all, only a base and a multiplier, so converting them requires a logarithm and can never be exact. Parts per million, percentages and ratios are dimensionless. Trade units such as the barrel of oil are exact but purely conventional.

Examples: decibel, neper, cent, part per million, oil barrel