SI Base Units

The metric international system of measurements (SI system) has only seven base units. These seven base units are sufficient to express all measurable quantities. They are listed below. For each quantity there also exist a wide variety of other non-SI units. If you click on the hyperlinks, you can see the conversion factors between the SI and non-SI units.

To each SI base unit you can add prefixes to generate other valid SI units.

Quantity

Name of SI unit

Symbol

Definition

Length

Meter

m

1 meter is the distance that light can cross in vacuum during a time of 1/299792458 fraction of a second

Mass

Kilogram

kg

Mass of the international prototype of the kilogram, stored in Paris

Time

Second

s

The time it takes for 9192631770 periods of the oscillation of light emitted in the transition between the two hyperfine levels of the ground state of ¹³³Cs atoms.

Current

Ampere

A

Strength of a constant electrical current, which, if flowing through 2 parallel infinitely long and infinitesimally thick wires at 1 m distance from each other, would generate a force of 2·10^-7 N per meter length.

Temperature

Kelvin

K

A fraction of 273.16 of the temperature interval between the triple point of water and absolute 0 temperature.

Quantity

Mole

mol

Amount of material that has as many atoms or molecules as there are atoms in 0.012 kg of ¹²C.

Light strength

Candela

cd

Strength of a radiation source, which emits monochromatic radiation of frequency 5.40·10¹⁴ Hz, and which has a power of 1/683 Watts per steradiant.

Quantity	Name of SI unit	Symbol	Definition
Length	Meter	m	1 meter is the distance that light can cross in vacuum during a time of 1/299792458 fraction of a second
Mass	Kilogram	kg	Mass of the international prototype of the kilogram, stored in Paris
Time	Second	s	The time it takes for 9192631770 periods of the oscillation of light emitted in the transition between the two hyperfine levels of the ground state of ¹³³Cs atoms.
Current	Ampere	A	Strength of a constant electrical current, which, if flowing through 2 parallel infinitely long and infinitesimally thick wires at 1 m distance from each other, would generate a force of 2·10^-7 N per meter length.
Temperature	Kelvin	K	A fraction of 273.16 of the temperature interval between the triple point of water and absolute 0 temperature.
Quantity	Mole	mol	Amount of material that has as many atoms or molecules as there are atoms in 0.012 kg of ¹²C.
Light strength	Candela	cd	Strength of a radiation source, which emits monochromatic radiation of frequency 5.40·10¹⁴ Hz, and which has a power of 1/683 Watts per steradiant.