The International System of Units – A Complete Guide to the SI
The International System of Units, also commonly referred to as the SI, is a system of weights and measures. The International System of Units is the modern version of the metric system.
The SI is the preferred and most commonly used system of measurement in use today.
What is the SI?
The SI is a system of measurement built on the previous incarnation of the metric system or the metre–kilogram–second system, or MKS. It was formed to resolve inconsistencies between the MKS and the centimetre–gram–second system, or CGS.
The SI was formed by international agreement in 1960, and it builds on the MKS and adds additional units that serve as the base units from which all other units are derived.
Units of the SI
The SI is composed of 7 base units of measure, which are used to derive an additional 22 units with special names.
Base units are the foundational units that are used to define other units in the SI. Refer to the table below, showing the seven base units of the SI.
|Base Quantity||Base Unit||Symbol|
|amount of substance||mole||mol|
The SI defines several units that are derived from the base units using simple equations, generally the product of the powers of base units.
|Derived Quantity||Derived Unit||Symbol|
|speed, velocity||meter per second||m/s|
|acceleration||meter per second squared||m/s2|
|wave number||reciprocal meter||m-1|
|mass density||kilogram per cubic meter||kg/m3|
|specific volume||cubic meter per kilogram||m3/kg|
|current density||ampere per square meter||A/m2|
|magnetic field strength||ampere per meter||A/m|
|amount-of-substance concentration||mole per cubic meter||mol/m3|
|luminance||candela per square meter||cd/m2|
SI Derived Units with Special Names
The SI also defines 22 units with special names and symbols that are also derived from base units. These derived units with special names can be used with SI base units and derived units.
|Derived Quantity||Derived Unit||Symbol||Expression|
|energy, work, quantity of heat||joule||J||m2·kg·s-2|
|power, radiant flux||watt||W||m2·kg·s-3|
|electric charge, quantity of electricity||coulomb||C||s·A|
|electric potential difference, electromotive force||volt||V||m2·kg·s-3·A-1|
|magnetic flux density||tesla||T||kg·s-2·A-1|
|Celsius temperature||degree Celsius||°C||K|
|activity (of a radionuclide)||becquerel||Bq||s-1|
|absorbed dose, specific energy (imparted), kerma||gray||Gy||m2·s-2|
SI Prefixes and Multiples
The SI also defines prefixes that can be used to express very small or very large numerical values that are multiples or submultiples of the base quantity. The prefixes are placed directly in front of the unit name and express the multiple of the unit.
For example, one kilometer is equal to 1,000 meters. The prefix “kilo” indicates that the value is equivalent to the base unit times 103.
Multiplying a coefficient by 10n is also referred to as scientific notation form. You can use our scientific notation calculator to convert a value expressed using an SI prefix with the multiple to decimal.
The table below shows the complete list of SI prefixes.
|1024||yotta||Y||1 000 000 000 000 000 000 000 000|
|1021||zetta||Z||1 000 000 000 000 000 000 000|
|1018||exa||E||1 000 000 000 000 000 000|
|1015||peta||P||1 000 000 000 000 000|
|1012||tera||T||1 000 000 000 000|
|109||giga||G||1 000 000 000|
|106||mega||M||1 000 000|
|10–9||nano||n||0.000 000 001|
|10–12||pico||p||0.000 000 000 001|
|10–15||femto||f||0.000 000 000 000 001|
|10–18||atto||a||0.000 000 000 000 000 001|
|10–21||zepto||z||0.000 000 000 000 000 000 001|
|10–24||yocto||y||0.000 000 000 000 000 000 000 001|
What Does SI Stand For?
The SI is the abbreviation for Système Internationale, or Système Internationale d’Unités, which is the French name for the International System of Units. Since the International System of Units originated in France, the French abbreviation is used.
Why do we use SI Units?
The SI system provides a standardized and consistent set of units. Because they are derived from a consistent set of base units and multiples are predictable using standard prefixes, SI units are ideal for use in science and technology.
SI units reduce variation in conversions between units, and thus, using them reduces error in calculations. SI units are also very easy to work with and simplify complex equations.
SI units are also preferable for use due to their widespread adoption and standardization. Because they are used across the world, sharing information and data expressed in SI units is much simpler.
Who Oversees the SI?
The International Bureau of Weights and Measures, or the Bureau International des Poids et Mesures in French, oversees the International System of Units. Also known as the BIPM, the International Bureau of Weights and Measures is an intergovernmental organization that was founded by international agreement in 1875 as part of the signing of the Metre Convention.
Our unit conversion calculators allow you to convert to and from the SI units of measure and other measurement systems, such as US Customary or CGS units.
- Bureau International des Poids et Mesures, The International System of Units (SI), 9th edition, 2019, https://www.bipm.org/documents/20126/41483022/SI-Brochure-9-EN.pdf
- National Institute of Standards and Technology, SI Units, https://www.nist.gov/pml/owm/metric-si/si-units
- David B. Newell and Eite Tiesinga, NIST Special Publication 330, The International System of Units (SI), 2019 Edition, National Institute of Standards and Technology, https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.330-2019.pdf
- Ambler Thompson and Barry N. Taylor, Guide for the Use of the International System of Units (SI), NIST Special Publication 811, 2008 Edition, National Institute of Standards and Technology, https://physics.nist.gov/cuu/pdf/sp811.pdf
- The Metre Convention, Signed in Paris on 20th May 1875 Modified by the Convention Signed at Sèvres on 6th October 1921 and Annexed Regulations, https://www.bipm.org/documents/20126/44107685/metre-convention.pdf/cd9c9e57-0cc6-4cda-2930-ec1d7c853743