- French:
- Système International d’Unités
- Related Topics:
- mole
- kilogram
- second
- metric ton
- nanometre
- On the Web:
- Biology LibreTexts - The International System of Units (Oct. 21, 2026)
International System of Units
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What is the International System of Units (SI)?
The International System of Units (SI) is the international decimal system of weights and measures derived from the metric system. It was established in 1960 by the General Conference on Weights and Measures (CGPM) and is abbreviated SI in all languages. Physicists and scientists use SI units because they provide a globally agreed-upon system. Since 2019, SI units have been defined using fundamental physical constants, ensuring that scientists worldwide can agree on measurements of physical phenomena. The SI builds upon the meter-kilogram-second (MKS) system. The base units include:
- Metre (m) for length
- Kilogram (kg) for mass
- Second (s) for time
- Ampere (A) for electric current
- Kelvin (K) for thermodynamic temperature
- Candela (cd) for luminous intensity
- Mole (mol) for amount of substance
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Why is it important to have a standard system for measurement?
Having a standard system for measurement, like the International System of Units (SI), is important because it ensures uniformity, which is essential for associating numbers with physical quantities and phenomena. The SI is a planned system of units based on the metric system that is used by the world’s scientific community and most nations. It provides accurate and reliable standards for measurements, such as mass, length, area, and volume. This allows scientists anywhere to agree upon the units they use to measure physical phenomena.
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What are the basic units in the SI system?
The International System of Units (SI) is the standard international decimal system for weights and measures. It was established in 1960 and is abbreviated SI in all languages.
The SI builds upon the meter-kilogram-second (MKS) system. The seven base units are:
* Meter (m) for length
* Kilogram (kg) for mass
* Second (s) for time
* Ampere (A) for electric current
* Kelvin (K) for thermodynamic temperature
* Candela (cd) for luminous intensity
* Mole (mol) for amount of substanceSince 2019, SI units have been defined using fundamental physical constants. Because scientists worldwide agree upon the system, they can agree on the units they use to measure physical phenomena.
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How are length, mass, and time measured in the SI system?
The International System of Units (SI) defines units for length, mass, and time based on international agreements.
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Length The SI unit of length is the metre (m), defined as the distance traveled by light in a vacuum in 1⁄299,792,458 of a second.
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Mass The SI unit for mass is the kilogram (kg). Since 2019, the kilogram has been defined using Planck's constant, a fundamental physical constant.
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Time The SI unit of time is the second (s), defined by the duration of 9,192,631,770 cycles of radiation associated with a specific energy transition of the cesium-133 atom.
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What are prefixes like kilo- and milli- used for in the SI system?
Prefixes like kilo- and milli- in the metric system (part of the International System of Units (SI)) denote the factor by which the base unit is multiplied.
- Kilo- indicates a multiplication by 1,000. For example, 1 kilometer is 1,000 meters.
- Milli- indicates a multiplication by 0.001. For example, 1 millimeter is 0.001 meters.
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How does the SI system help scientists and people around the world communicate measurements accurately?
The International System of Units (SI) is a system that is based on the metric system and was created for scientific work. Physicists and scientists use the SI system because they want to use a system agreed upon by scientists worldwide.
Since 2019, SI units have been defined using fundamental physical constants. This allows scientists to agree on units for measuring physical phenomena. The widely used units in the SI system include metre, second, kilogram, ampere, kelvin, candela, and mole.
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International System of Units (SI), international decimal system of weights and measures derived from and extending the metric system of units. Adopted by the 11th General Conference on Weights and Measures (CGPM) in 1960, it is abbreviated SI in all languages.
Rapid advances in science and technology in the 19th and 20th centuries fostered the development of several overlapping systems of units of measurements as scientists improvised to meet the practical needs of their disciplines. The early international system devised to rectify this situation was called the metre-kilogram-second (MKS) system. The CGPM added three new units (among others) in 1948: a unit of force (the newton), defined as that force which gives to a mass of one kilogram an acceleration of one metre per second per second; a unit of energy (the joule), defined as the work done when the point of application of a newton is displaced one metre in the direction of the force; and a unit of power (the watt), which is the power that in one second gives rise to energy of one joule. All three units are named for eminent scientists.
The 1960 International System builds on the MKS system. Its seven basic units, from which other units are derived, were defined as follows: for length, the metre, defined as the distance traveled by light in a vacuum in 1/299,792,458 second; for mass, the kilogram, which equaled 1,000 grams as defined by the international prototype kilogram of platinum-iridium in the keeping of the International Bureau of Weights and Measures in Sèvres, France; for time, the second, the duration of 9,192,631,770 periods of radiation associated with a specified transition of the cesium-133 atom; for electric current, the ampere, which was the current that, if maintained in two wires placed one metre apart in a vacuum, would produce a force of 2 × 10−7 newton per metre of length; for luminous intensity, the candela, defined as the intensity in a given direction of a source emitting radiation of frequency 540 × 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian; for amount of substance, the mole, defined as containing as many elementary entities of a substance as there are atoms in 0.012 kg of carbon-12; and for thermodynamic temperature, the kelvin.
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On May 20, 2019, the CGPM redefined the kilogram, the ampere, the mole, and the kelvin in terms of fundamental physical constants. For the kilogram, the constant chosen was Planck’s constant, which is defined as equal to 6.62607015 × 10−34 joule second. One joule is equal to one kilogram times metre squared per second squared. Since the second and the metre were already defined, the kilogram would then be determined by accurate measurements of Planck’s constant. The ampere was redefined such that the elementary charge is equal to 1.602176634 × 10−19 coulomb. The kelvin was redefined such that the Boltzmann constant is equal to 1.380649 × 10−23 joule per kelvin, and the mole was redefined such that the Avogadro constant is equal to 6.02214076 × 1023 per mole.
Widely used units in the SI system
A list of the widely used units in the SI system is provided in the table.
| unit | abbreviation | physical quantity | ||
|---|---|---|---|---|
| Base units | metre | m | length | |
| second | s | time | ||
| kilogram | kg | mass | ||
| ampere | A | electric current | ||
| kelvin | K | thermodynamic temperature | ||
| candela | cd | luminous intensity | ||
| mole | mol | amount of substance | ||
| unit | abbreviation | number of metres | approximate U.S. equivalent | |
| Length | kilometre | km | 1,000 | 0.62 mile |
| centimetre | cm | 0.01 | 0.39 inch | |
| millimetre | mm | 0.001 | 0.039 inch | |
| micrometre | μm | 0.000001 | 0.000039 inch | |
| nanometre | nm | 0.000000001 | 0.000000039 inch | |
| unit | abbreviation | number of square metres | approximate U.S. equivalent | |
| Area | square kilometre | sq km, or km2 | 1,000,000 | 0.3861 square mile |
| hectare | ha | 10,000 | 2.47 acres | |
| are | a | 100 | 119.60 square yards | |
| square centimetre | sq cm, or cm2 | 0.0001 | 0.155 square inch | |
| unit | abbreviation | number of cubic metres | approximate U.S. equivalent | |
| Volume | cubic metre | m3 | 1 | 1.307 cubic yards |
| cubic centimetre | cu cm, cm3, or cc | 0.000001 | 0.061 cubic inch | |
| unit | abbreviation | number of litres | approximate U.S. equivalent | |
| Capacity | kilolitre | kl | 1,000 | 1.31 cubic yards |
| litre | l | 1 | 61.02 cubic inches | |
| centilitre | cl | 0.01 | 0.61 cubic inch | |
| millilitre | ml | 0.001 | 0.061 cubic inch | |
| microlitre | μl | 0.000001 | 0.000061 cubic inch | |
| unit | abbreviation | number of grams | approximate U.S. equivalent | |
| Mass and weight | metric ton | t | 1,000,000 | 1.102 short tons |
| gram | g | 1 | 0.035 ounce | |
| centigram | cg | 0.01 | 0.154 grain | |
| milligram | mg | 0.001 | 0.015 grain | |
| microgram | μg | 0.000001 | 0.000015 grain | |
| unit | symbol | physical quantity | expressed in base units | |
| Energy | hertz | Hz | frequency | 1/s |
| newton | N | force, weight | (m × kg)/s2 | |
| joule | J | work, energy, quantity of heat | (m2 × kg)/s2 | |
| pascal | Pa | pressure, stress | kg/(m × s2) | |
| watt | W | power | (m2 × kg)/s3 | |
| coulomb | C | electric charge | s × A | |
| volt | V | electric potential difference | (m2 × kg)/(s3 × A) | |
| farad | F | electric capacitance | (s2 × s2 × A2)/(m2 × kg) | |
| ohm | Ω | electric resistance, reactance | (m2 × kg)/(s3 × A2) | |
| siemens | S | electric conductance | (s3 × A2)/(m2 × kg) | |
| weber | Wb | magnetic flux | (m2 × kg)/(s2 × A) | |
| tesla | T | magnetic induction | kg/(s2 × A) | |
| henry | H | inductance | (m2 × kg)/(s2 × A2) | |
| lumen | lm | luminous flux | cd × sr | |
| lux | lx | illuminance | (cd × sr)/m2 | |
Metric conversions
A list of metric conversions is provided in the table.
| approximate common equivalents | |
|---|---|
| *Common term not used in SI. | |
| **Exact. | |
| Source: National Bureau of Standards Wall Chart. | |
| 1 inch | = 25 millimetres |
| 1 foot | = 0.3 metre |
| 1 yard | = 0.9 metre |
| 1 mile | = 1.6 kilometres |
| 1 square inch | = 6.5 square centimetres |
| 1 square foot | = 0.09 square metre |
| 1 square yard | = 0.8 square metre |
| 1 acre | = 0.4 hectare* |
| 1 cubic inch | = 16 cubic centimetres |
| 1 cubic foot | = 0.03 cubic metre |
| 1 cubic yard | = 0.8 cubic metre |
| 1 quart (liq) | = 1 litre* |
| 1 gallon | = 0.004 cubic metre |
| 1 ounce (avdp) | = 28 grams |
| 1 pound (avdp) | = 0.45 kilogram |
| 1 horsepower | = 0.75 kilowatt |
| 1 millimetre | = 0.04 inch |
| 1 metre | = 3.3 feet |
| 1 metre | = 1.1 yards |
| 1 kilometre | = 0.6 mile (statute) |
| 1 square centimetre | = 0.16 square inch |
| 1 square metre | = 11 square feet |
| 1 square metre | = 1.2 square yards |
| 1 hectare* | = 2.5 acres |
| 1 cubic centimetre | = 0.06 cubic inch |
| 1 cubic metre | = 35 cubic feet |
| 1 cubic metre | = 1.3 cubic yards |
| 1 litre* | = 1 quart (liq) |
| 1 cubic metre | = 264 gallons |
| 1 gram | = 0.035 ounce (avdp) |
| 1 kilogram | = 2.2 pounds (avdp) |
| 1 kilowatt | = 1.3 horsepower |
| conversions accurate within 10 parts per million | |
| inches × 25.4** | = millimetres |
| feet × 0.3048** | = metres |
| yards × 0.9144** | = metres |
| miles × 1.60934 | = kilometres |
| square inches × 6.4516** | = square centimetres |
| square feet × 0.0929030 | = square metres |
| square yards × 0.836127 | = square metres |
| acres × 0.404686 | = hectares |
| cubic inches × 16.3871 | = cubic centimetres |
| cubic feet × 0.0283168 | = cubic metres |
| cubic yards × 0.764555 | = cubic metres |
| quarts (liq) × 0.946353 | = litres |
| gallons × 0.00378541 | = cubic metres |
| ounces (avdp) × 28.3495 | = grams |
| pounds (avdp) × 0.453592 | = kilograms |
| horsepower × 0.745700 | = kilowatts |
| millimetres × 0.0393701 | = inches |
| metres × 3.28084 | = feet |
| metres × 1.09361 | = yards |
| kilometres × 0.621371 | = miles (statute) |
| square centimetres × 0.155000 | = square inches |
| square metres × 10.7639 | = square feet |
| square metres × 1.19599 | = square yards |
| hectares × 2.47105 | = acres |
| cubic centimetres × 0.0610237 | = cubic inches |
| cubic metres × 35.3147 | = cubic feet |
| cubic metres × 1.30795 | = cubic yards |
| litres × 1.05669 | = quarts (liq) |
| cubic metres × 264.172 | = gallons |
| grams × 0.0352740 | = ounces (avdp) |
| kilograms × 2.20462 | = pounds (avdp) |
| kilowatts × 1.34102 | = horsepower |