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 0^{o}C 
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 10^{12} 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/s^{2}) 
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/m^{2}) 
Luminous flux 

lumen 

lm 

Candela steradians (cd.sr) 
Illuminance 

lux 

lx 

Lumens per square metre (lm/m^{2}) 
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/m^{2}), or webers per square metre (Wb/m^{2}) 
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) 
Thermodynamic temperature 

degree Celsius 

^{o}C 

Kelvins minus 273.15 (K – 273.15) 
Other quantities and units (selected)
Quantity 

Expressed as 

Abbr 
Area 

Square metre 

m^{2} 
Volume 

Cubic metre 

m^{3} 
Speed, velocity 

Metre per second 

m/s 
Acceleration 

Metre per second squared 

m/s^{2} 
Jerk 

Metre per second cubed 

m/s^{3} 
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/m^{3} 
Specific volume 

Cubic metre per kilogram 

m^{3}/kg 
Heat capacity, entropy 

Joule per kelvin 

J/K 
Surface tension 

Newton per metre 

N/m or J/m^{2} 
Thermal conductivity 

Watt per metre kelvin 

W/(m.k) 
Kinematic viscosity 

Square metre per second 

m^{2}/s 
Dynamic viscosity 

Pascal second 

Pa.s or N.s/m^{2} 
Conductivity 

Siemens per metre 

S/m 
Luminance 

Candela per square metre 

Cd/m^{2} 
Exposure (to X and gamma rays) 

Coulomb per kilogram 

C/kg 
Absorbed dose rate 

Gray per second 

Gy/s 
© Haydn Thompson 2017