Fundamentals of SI Units

The foundation of modern metrology is the International System of Units (SI), which is used internationally to define the fundamental units of measurement. These are used to derive other units and are used to define other measurement units, such as may be used locally.

The SI consists of 28 units:

7 Base Units
2 Supplementary Units
19 Derivative Units

These are defined in the tables below for reference.

Base Units

Parameter	SI Unit	Abbreviation	Definition
Length	metre	m	The distance travelled by light in a vacuum in 1/299,792,458 s.
Mass	kilogram	kg	The mass of a cylinder of platinum iridium alloy kept in Paris, France.
Time	second	s	The duration of 9,192,631,770 cycles of radiation corresponding to a particular transition of the caesium-133 atom.
Electric Current	ampere	A	The current producing a force of 2*10^-7 newtons per metre of length between two long wires 1 m apart in space.
Thermodynamic Temperature	kelvin	K	1/273.16 of the thermodynamic triple point of water.
Luminous Intensity	candela	cd	The luminous intensity in a given direction of a source that emits monochromatic radiation at a frequency of 540*10¹² hertz, with a radiant intensity in that direction of 1/683 watts per steradian.
Amount of Substance	mole	m	The amount of substance in a system that contains as many elementary items as there are atoms in 0.012 kg of carbon 12.

Supplementary Units

Parameter	SI Unit	Abbreviation	Definition
Plane Angles	radian	rad	A plane angle with vertex at the center of a circle that is subtended by an arc equal in length to the radius.
Solid Angles	steradian	sr	The solid angle with vertex at the center of a sphere that is subtended by an area of the spherical circle equal to that of a square with sides equal in length to the radius.

Derivative Units

Parameter	SI Unit	Abbreviation	Definition
Frequency	hertz	Hz	1/s
Force	newton	N	kg*m/s²
Pressure	pascal	Pa	N/m²
Work or Energy	joule	J	N*m
Power	watt	W	J/s
Electric Potential	volt	V	W/A
Electric Resistance	ohm	ohm	V/A
Quantity of Charge	coulomb	C	A*s
Electric Capacitance	farad	F	C/V
Conductance	siemens	S	A/V
Magnetic Flux	weber	Wb	V*s
Magnetic Flux Density	tesla	T	Wb/²
Inductance	henry	H	Wb/A
Celsius Temperature	degree	°C	K
Luminous Flux	lumen	lm	cd*sr
Illuminance	lux	lx	lm/m²
Activity	becquerel	Bq	1/s
Absorbed Dose	gray	Gy	J/kg
Dose Equivalent	sievert	Sv	m²/s²