The International System of Units, abbreviated SI from the French Système international d'unités, is the measurement system used by science everywhere and by law in almost every country. It was adopted in 1960 and is maintained by the International Bureau of Weights and Measures near Paris. It defines seven base units, a set of derived units built from them, and a range of decimal prefixes that scale any unit up or down by powers of ten.
Two features set SI apart from older systems. The first is that it is decimal throughout: every step between related units is a power of ten, so converting within SI moves a decimal point rather than multiplying by 12, 14 or 1760. The second is that since 2019 no SI unit depends on a physical object. Each one is defined by fixing the numerical value of a constant of nature, which means the definitions can be reproduced in any properly equipped laboratory and will not drift.
The metric system was created in France in the 1790s. The revolutionary government wanted units that were not the property of a king or a guild, so it based them on the Earth itself. The metre was defined as one ten-millionth of the distance from the North Pole to the equator along the meridian through Paris. The gram followed as the mass of one cubic centimetre of water. Both definitions were replaced by physical prototypes in 1799 because measuring the meridian was impractical.
The Metre Convention of 1875 made the system international. Seventeen states signed it and created the International Bureau of Weights and Measures, which produced new platinum-iridium prototypes of the metre and the kilogram and distributed copies to member states. For most of the following century, the metre and the kilogram were physical bars and cylinders held in a vault at Sèvres.
The SI itself was established in 1960 by the General Conference on Weights and Measures. It selected six base units, added the mole in 1971 to make seven, and organised the derived units and prefixes into the coherent structure used today. The word "coherent" matters here: derived units are formed from base units with no numerical factors, so a joule is exactly one kilogram metre squared per second squared, and no conversion constant appears anywhere in the system.
Every SI unit is built from seven base units. Each is now defined by fixing the value of a constant, shown in the table below.
| Quantity | Unit | Symbol | Defined by fixing |
|---|---|---|---|
| Time | second | s | the caesium-133 hyperfine frequency |
| Length | metre | m | the speed of light, c |
| Mass | kilogram | kg | the Planck constant, h |
| Electric current | ampere | A | the elementary charge, e |
| Temperature | kelvin | K | the Boltzmann constant, k |
| Amount of substance | mole | mol | the Avogadro constant, NA |
| Luminous intensity | candela | cd | the luminous efficacy Kcd |
The kilogram is the odd one out in naming: it is the only base unit whose name carries a prefix, a historical accident from the gram being chosen first and then found too small for practical standards.
Until 2019 the kilogram was still a physical object, a platinum-iridium cylinder made in 1889. Comparisons over a century showed that the prototype and its official copies were drifting apart by tens of micrograms. Since the prototype was the kilogram by definition, it could not itself be wrong, so the mass of every other object in the world was formally changing instead.
The 2019 revision removed the last artefact. The kilogram is now defined by fixing the Planck constant at exactly 6.62607015 × 10-34 joule seconds, realised in practice with a Kibble balance that weighs a mass against an electromagnetic force. At the same time the ampere, the kelvin and the mole were redefined in terms of the elementary charge, the Boltzmann constant and the Avogadro constant. The practical sizes of the units did not change; the redefinition was designed so that no measurement in ordinary use would shift.
Only seven units are defined outright. Everything else in SI is derived from them by multiplication and division, with no numerical factor introduced at any point. Speed is metres per second. Density is kilograms per cubic metre. Acceleration is metres per second squared. These have no special names and are simply written as combinations.
Twenty-two derived units do have their own names, granted because the combination is common enough to deserve shorthand. The newton is a kilogram metre per second squared. The joule is a newton metre. The watt is a joule per second. The pascal is a newton per square metre. The hertz is one per second. Each of these can be expanded back into base units without any conversion constant, which is what makes the system coherent and is the reason physics equations in SI contain no unit-conversion factors.
The practical benefit shows up in calculation. If a problem is stated entirely in SI base and derived units, the answer comes out in SI units automatically. There is no step where a figure must be multiplied by 12, or by 1.8, or by 4.546 to keep the quantities compatible. Systems that lack this property require conversion constants inside the equations themselves, which is a common source of error.
Prefixes are what make SI convenient across scales. Each is a fixed power of ten and can be attached to any unit. The system spans thirty orders of magnitude in each direction, extended in 2022 with ronna, quetta, ronto and quecto to accommodate data storage and particle physics.
| Prefix | Symbol | Factor | Prefix | Symbol | Factor |
|---|---|---|---|---|---|
| kilo | k | 103 | milli | m | 10-3 |
| mega | M | 106 | micro | µ | 10-6 |
| giga | G | 109 | nano | n | 10-9 |
| tera | T | 1012 | pico | p | 10-12 |
| peta | P | 1015 | femto | f | 10-15 |
| exa | E | 1018 | atto | a | 10-18 |
Capitalisation carries meaning and is not optional. A capital M is mega and a lowercase m is milli, so MJ and mJ differ by a factor of a billion. The micro prefix uses the Greek letter mu. Because URLs and plain text often cannot preserve that distinction, this site treats a lowercase abbreviation as the milli unit and requires the exact symbol for mega and micro.
The converters here cover the SI base units and the derived and prefixed units built on them. The most used are listed below.
| Quantity | SI unit | Convert |
|---|---|---|
| Length | metre | meters to feet |
| Mass | kilogram | kilograms to pounds |
| Temperature | kelvin | kelvins to celsius |
| Time | second | seconds to minutes |
| Energy | joule | joules to calories |
| Pressure | pascal | pascals to bars |
| Frequency | hertz | hertz to kilohertz |
| Area | square metre | square meters to square feet |
| Volume | cubic metre | cubic meters to liters |
Two units in that list sit slightly outside strict SI. The litre is not an SI unit but is accepted for use with it, and it is defined as exactly one cubic decimetre. Degrees Celsius are likewise accepted: the degree is the same size as the kelvin, offset by 273.15, so a temperature difference is identical in both while an absolute temperature is not.
Conversions within SI never lose precision, because every step is a power of ten. Converting metres to millimetres multiplies by exactly 1000. Conversions out of SI are also exact wherever the other system defines itself in metric terms, which since 1959 includes the Imperial and US customary units of length and mass. An inch is exactly 25.4 millimetres, not approximately.
The exceptions are units defined by measurement rather than agreement. The astronomical unit and the electronvolt depend on measured physical quantities, so their metric values carry uncertainty in their final digits. For everyday conversion that uncertainty is far below any practical significance.
The SI has a written style, and it is stricter than most people expect. Unit symbols are never pluralised: five metres is 5 m, never 5 ms, which would mean five milliseconds. Symbols take no full stop unless they end a sentence. A space separates the number from the symbol, so it is 25 kg rather than 25kg, and the same applies to the degree symbol in 25 °C.
Capitalisation follows one rule. Unit names are written in lowercase even when they come from a person's name, so the unit is the newton and the pascal, not the Newton and the Pascal. The symbol, by contrast, is capitalised when the unit is named after a person: N for newton, Pa for pascal, W for watt, K for kelvin. Units not named after people take lowercase symbols, such as m, s and kg. The litre is a special case, permitting both l and L because a lowercase l is easily mistaken for a digit one.
Prefixes attach directly to the unit with no space and no hyphen, and only one prefix may be used at a time. A millionth of a kilogram is a milligram, never a microkilogram. Prefix symbols above kilo are capitalised, which is why M means mega while m means milli, and getting that wrong changes a quantity by a factor of a billion.
Three countries have not adopted SI as their primary system: the United States, Liberia and Myanmar. Even there, SI dominates science, medicine and international trade. Elsewhere the exceptions are narrow and cultural rather than legal. The United Kingdom keeps miles on road signs and pints in pubs. Aviation and shipping worldwide use nautical miles, knots and often feet for altitude. Computing measures data in bits and bytes with binary as well as decimal prefixes.
You can browse every unit on this site from the unit dictionary, or compare SI against the Imperial system and the US customary system.