SI Base Units

The International System of Units (SI) define the following base units


Meter (m)
The length of the path traveled by light in a vacuum during a time interval of 1/299 792 458 of a second.


Kilogram (kg)
Equal to the mass of the international prototype of the kilogram. The prototype is an alloy of 90% platinum and 10% iridium. It was manufactured in the 1880s and kept with its six official copies in a vault at the BIPM.


Second (s)
The base unit of time. It is the duration of 9 192 631 770 periods of radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.


Ampere (A)
A constant electric current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in a vacuum, would produce between these conductors a force equal to 2 × 10-7 newton per meter of length.


Kelvin (K)
The fraction 1/273.16 of the thermodynamic temperature of the triple point of water.


Candela (cd)
Unit for luminous intensity. A source that emits monochromatic radiation of frequency 540 × 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.


Mole (mol)
Unit for quantity of matter. The amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12.