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.

SI logo showing symbols for the 7 base units of measure
7 SI base units, source: BIPM.org, licensed under CC BY-ND 4.0

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.[1]

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.[2]

Base Units

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.

SI base units and their symbols
Base Quantity Base Unit Symbol
length meter m
mass kilogram kg
time second s
electric current ampere A
thermodynamic temperature kelvin K
amount of substance mole mol
luminous intensity candela cd

Derived Units

The SI defines several units that are derived from the base units using simple equations, generally the product of the powers of base units.[3]

SI derived units and their symbols
Derived Quantity Derived Unit Symbol
area square meter m2
volume cubic meter m3
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.

SI derived units with special names and their expressions
Derived Quantity Derived Unit Symbol Expression
plane angle radian rad m·m-1
solid angle steradian sr m2·m-2
frequency hertz Hz s-1
force newton N m·kg·s-2
pressure, stress pascal Pa m-1·kg·s-2
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
capacitance farad F m-2·kg-1·s4·A2
electric resistance ohm Ω m2·kg·s-3·A-2
electric conductance siemens S m-2·kg-1·s3·A2
magnetic flux weber Wb m2·kg·s-2·A-1
magnetic flux density tesla T kg·s-2·A-1
inductance henry H m2·kg·s-2·A-2
Celsius temperature degree Celsius °C K
luminous flux lumen lm cd
illuminance lux lx m-2·cd
activity (of a radionuclide) becquerel Bq s-1
absorbed dose, specific energy (imparted), kerma gray Gy m2·s-2
dose equivalent sievert Sv m2·s-2
catalytic activity katal kat s-1·mol

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.[4]

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.

Refer to the table below for the complete list of SI prefixes.

SI prefixes and the equivalent factors and multiples
Factor Prefix Symbol Multiple
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
103 kilo k 1 000
102 hecto h 100
101 deca da 10
10–1 deci d 0.1
10–2 centi c 0.01
10–3 milli m 0.001
10–6 micro µ 0.000 001
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.[5]

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.

References

  1. Bureau International des Poids et Mesures, The International System of Units (SI), 9th edition, 2019, https://www.bipm.org/utils/common/pdf/si-brochure/SI-Brochure-9-EN.pdf
  2. National Institute of Standards and Technology, SI Units, https://www.nist.gov/pml/weights-and-measures/metric-si/si-units
  3. 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
  4. 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
  5. Bureau International des Poids et Mesures, History of the SI, https://www.bipm.org/en/measurement-units/history-si/