SI Units

SI stands for Système international. The International System of Units (French Système international d'unités is, as its name implies, the international standard for metric units. It's based on the metre, kilogram and second – and is sometimes referred to by the abbreviation MKS, to distinguish it from the earlier CGS system which was based on the centimetre, gram and second.

The most common type of question in this category is "What is the SI unit of ... ?"

This page provides a little more background information.

Base Units

These are the seven basic SI units from which all the others are derived.

Quantity		Unit		Abbr	Definition
Length		Metre		m	The distance travelled by light in a vacuum in a specified time. Historically, 10-7 times the distance from the North Pole to the Equator, measured through Paris; in other words, that distance was exactly 10,000 kilometres
Mass		Kilogram		kg	The weight of a specific cylinder of platinum–iridium, kept at the International Bureau of Weights and Measures in Paris. Historically, the weight of one litre of water
Time		Second		s	Defined according to the radiation period of a Caesium–133 atom at 0oC
Electrical current		Ampere		A	Defined according to the force (10-7 newtons) produced between two conductors one metre apart by this amount of current flowing in each conductor
Thermodynamic temperature		Kelvin		K	1/273.16 of the thermodynamic (absolute) temperature at the triple point of water. Historically, 1/100th of the difference between the freezing and boiling points of water
Quantity of matter (mass)		Mole		mol	The quantity of substance that contains the same number of elementary entities (atoms, molecules, ions, electrons or particles, depending on the substance) as there are atoms in 0.012 kilograms of pure carbon–12; approximately equal to 6 x 1023; historically, one gram per atomic mass unit
Luminous intensity		Candela		cd	The 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. Based on the amount of light emitted by a standard candle (1 candlepower)

Dimensionless Derived Units

Quantity		Unit		Abbr	Definition
Angle		Radian		rad	The angle subtended at the centre of a circle by an arc of the circumference equal in length to the radius of the circle. There are 2π radians in a circle; a radian is equal to approximately 57.2°
Solid angle		Steradian		sr	The solid angle subtended at the centre of a sphere of radius r by a portion of the surface of the sphere having an area r2. There are 4π steradians on a sphere.

Derived Units

Quantity		Unit		Abbr		Expressed as
Frequency		hertz		Hz		Cycles per second (1/s)
Force, weight		newton		N		Metre kilograms per second squared (m.kg/s2)
Energy, work, heat		joule		J		Newton metres (N.m)
Power, radiant flux		watt		W		Joules per second (J/s)
Pressure, stress		pascal		Pa		Newtons per square metre (N/m2)
Luminous flux		lumen		lm		Candela steradians (cd.sr)
Illuminance		lux		lx		Lumens per square metre (lm/m2)
Electrical charge or flux		coulomb		C		Second amperes (s.A)
Electrical potential difference, electromotive force		volt		V		Watts per ampere(W/A), or joules per coulomb (J/C)
Electrical resistance, impedance, reactance		ohm		Ω		Volts per ampere (V/A)
Electrical capacitance		farad		F		Coulombs per volt (C/V)
Magnetic flux		weber		Wb		Joules per ampere (J/A)
Magnetic flux density, magnetic induction		tesla		T		Volt seconds per square metre (V.s/m2), or webers per square metre (Wb/m2)
Inductance		henry		H		Volt seconds per ampere (V.s/A), or webers per ampere (Wb/A)
Electrical conductance		siemens		S		Per ohm (1/Ω) – also known as the mho
Radioactivity		bequerel		Bq		Decays per second (1/s)
Absorbed dose of radiation		gray		Gy		Joules per kilogram (J/kg)
Equivalent dose of radiation		sievert		Sv		Joules per kilogram (J/kg)
Catalytic activity		katal		kat		Moles per second (mol/s)
Temperature relative to 273.15 Kelvin		degree Celsius		oC		Kelvins minus 273.15 (K – 273.15)

Other quantities and units (selected)

Quantity		Expressed as		Abbr
Area		Square metre		m2
Volume		Cubic metre		m3
Speed, velocity		Metre per second		m/s
Acceleration		Metre per second squared		m/s2
Jerk		Metre per second cubed		m/s3
Angular velocity		Radian per second		Rad/s
Momentum, impulse		Newton second		N.s
Angular momentum		Newton metre second		N.m.s
Torque, moment of force		Newton metre		N.m
Density, mass density		Kilogram per cubic metre		kg/m3
Specific volume		Cubic metre per kilogram		m3/kg
Heat capacity, entropy		Joule per kelvin		J/K
Surface tension		Newton per metre		N/m or J/m2
Thermal conductivity		Watt per metre kelvin		W/(m.k)
Kinematic viscosity		Square metre per second		m2/s
Dynamic viscosity		Pascal second		Pa.s or N.s/m2
Conductivity		Siemens per metre		S/m
Luminance		Candela per square metre		Cd/m2
Exposure (to X and gamma rays)		Coulomb per kilogram		C/kg
Absorbed dose rate		Gray per second		Gy/s

© Haydn Thompson 2017–22