SI Units | International System of Units

When we talk about the metric system, we are really referring to the SI system. SI deriving from the French "Système International d'Unités."
The SI system consists of 7 base units, and a number of derived units that have special names, e.g. watt.
Any of the SI units, base or derived, can be prefixed by multipliers, see our page on SI Prefixes for a list of common multipliers.

SI Base Units

The International System of Units defines seven units of measure as a basic set from which all other SI units are derived - these are known as the SI base units. The seven SI base units are:
meter (symbol m) is a measure of length. The meter has been defined as the distance light travels in a vacuum in exactly 1/299,792,458th of a second.
kilogram (symbol k) is a measure of mass. The kilogram is unique in being the only SI unit still defined by a physical prototype. It is equal to the mass of the International Prototype of the Kilogram, a platinum-iridium cylinder kept by the Bureau International des Poids et Mesures at Sévres, France, about 2.2046 pounds avoirdupois.
second (symbol s) is a measure of time. The second is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom.
ampere (symbol A) is a measure of electric current. One ampere is defined as that constant 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 x 10⁻⁷ newtons per meter of length.
kelvin (symbol K) is a measure of thermodynamic temperature. The kelvin is defined as 1/273.16 of the thermodynamic temperature of the triple point of water. Triple point being the temperature and pressure at which water can exist simultaneously as solid, liquid and gas. See this page on Temperature Conversion Formula for converting Kelvin into the more common temperature units used in daily life.
mole (symbol mol) is a measure of the amount of substance. One mole is defined as the amount of substance of a system that contains as many elementary entities as there are atoms in 0.012 kilograms of carbon-12.
candela (symbol cd) is a measure of luminous intensity. The candela is defined as the luminous intensity, in a given direction, of a [light] 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. A steradian being a measure of solid angle.

SI Derived Units

SI derived units are completely defined in terms of the base units above, but they have been given their own names.

Unit	Symbol	Property Measured	Definition
becquerel	BQ	radioactivity	One becquerel is defined as one spontaneous nuclear transition per second.
coulomb	C	electric charge	The amount of charge transported through any cross section of a conductor in one second by a constant current of one ampere. It is equivalent to the amount of charge on about 6,241,510,000,000,000,000 electrons.
degree Celsius	°C	temperature	Temperature on the Celsius scale is the temperature on the Kelvin scale minus 273.15
farad	F	capacitance	A capacitor has a capacitance of 1 farad if a charge of 1 coulomb increases the potential difference between its plates by 1 volt.
gray	Gy	absorbed dose	One gray is the quantity of ionising radiation that, absorbed in a mass of 1 kilogram, would impart to it 1 joule of energy.
henry	H	inductance	The self or mutual inductance of a closed loop is 1 henry if a current of 1 ampere gives rise to a magnetic flux of 1 weber.
hertz	Hz	frequency	The number of periods of a periodic phenomenon's frequency in 1 second.
joule	J	energy	The energy expended when the point of application of a force of 1 newton is displaced 1 meter in the direction of the force.
katal	kat	catalytic activity	One katal is that catalytic activity which will raise the rate of reaction by one mole per second in a specified assay system.
lumen	lm	luminous flux	One lumen is the luminous flux emitted within a solid angle of 1 steradian by a point source with an intensity of 1 candela.
lux	lx	illuminance	One lux is 1 lumen per square meter.
newton	N	force	A newton is that force which, applied to a mass of 1 kilogram, gives it an acceleration of 1 meter per second per second.
ohm	Ω	electrical resistance	The resistance of a circuit in which a potential difference of one volt produces a current of one ampere
pascal	Pa	pressure	One pascal is the pressure resulting from a force of 1 newton acting uniformly over an area of 1 square meter.
siemens	S	electric conductance	A conductor has a conductance of 1 siemens if an electrical potential difference of 1 volt produces a 1-ampere current in it. The reciprocal of resistance.
sievert	Sv	dose equivalent	A sievert of radiation is the amount of any kind of radiation which would cause the same amount of biological damage in a human being as would result from absorbing 1gray of X rays.
tesla	T	magnetic flux density	One tesla is one weber of magnetic flux per square meter of circuit area.
volt	V	electrical potential	One volt is the potential difference between two points on a conductor when the current flowing is one ampere and the power dissipated between the points is one watt.
watt	W	power	A watt is the power which in one second gives rise to energy of one joule.
weber	Wb	magnetic flux	One weber is the magnetic flux which, linking a circuit of one turn, would produce in it an electromotive force of 1 volt if it were reduced to zero at a uniform rate in 1 second.

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