SI Units

The International System of Units (SI, or Système International d'Unités), is a set of measurement standards which defines (almost) all standards in terms of uniform natural phenomena, and form the base of the metric system.

Base SI Units

There are seven base SI units, with many derived units made from combinations of these. Base SI units are mutually independent. They consist of:

Quantity Unit Abbreviation Definition
Time Second s Duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of an atom of Cs-133
Length Metre m Distance that light travels in a vacuum in 1/299,792,458th of a second
Current Ampere A The constant current that would produce a force of 2x10-7 Newton between two conductors of infinite length and negligible cross section in a vacuum
Temperature Kelvin °K 1/273.16th of the triple point of water. The triple point is the temperature at which a substance exists in equilibrium in all three phases (solid, liquid, gas).
Amount Mole mol The amount of substance which contains as many elementary entities as in 0.012kg of Carbon 12
Luminous Intensity Candella cd Luminous intensity of a source which emits monochromatic radiation at 540 x 1012 Hz at radiant intensity of 1/683 watts per steradian
Mass Kilogram kg Weight of the International Prototype Kilogram (IPK)

Derived Units

Quantity Unit Abbreviation Conversion to Base SI Units Definition
Area Square metre m2
Velocity Metre per Second m.s-1
Acceleration Metre per Second per Second m.s-2
Force Newton N Force required to accelerate 1kg at 1m.s-2
Pressure Pascal Pa Force per area
Energy/Work/Quantity of Heat Joule J Energy converted when 1N is applied to 1kg over 1m
Dose Equivalence Sievert Sv Radiation dose per mass
Power Watt W Rate of energy conversion per second
Electromotive Force Volt V Measure of electrical potential energy


  1. Physical Measurement Laboratory. National Institute of Standards and Technology.
    • And various subpages
Last updated 2017-09-22

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