Seven Base SI Units

Note that the symbols used for each unit are symbols, not abbreviations, and should not be capitalized or followed by a period. i.e. metre = m, not m., or M., and kilogram = kg, not K.g., KG., or KGr.

UnitPhysical QuantitySymbolDefinition
metrelengthmThe length of path travelled by light in a vacuum during the time interval of 1/299 792 458 of a second (17th CPGM, 1983);
originally 1/10 000 000 of the distance from the North Pole to the Equator through Paris, France.
kilogrammasskgThe mass equal to the International Prototype of the kilogram (3rd CPGM, 1901)
originally defined as the mass of 1 dm3 (1 litre) of water at 4°C.
secondtimesThe 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 (13th CGPM, 1967).
ampereelectric currentAThe constant of current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in a vacuum, would produce between these conductors a force equal to 2 x 10-7 newton per metre of length (9th CGPM, 1948).
kelvinthermodynamic temperatureKThe fraction 1/273.16 of the thermodynamic triple point of water (13th CGPM, 1967) (approximately the fraction 1/100 of temperature difference between the freezing point and boiling point of water at 101.315 kPa pressure).
moleamount of a substancemolThe amount of a substance of a system which contains as many elementary entities (atoms, molecules, ions, electrons, or other particles or specified groups of particles) as there are atoms in 0.012 kilogram of carbon-12. (14th CGPM, 1971).
candelaluminous intensitycdThe luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian (16th CGPM, 1979).

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