The International System of Units (abbreviated to SI from the French "Le Système International d'unités) is a metric system of units constructed from seven base units.
The following table gives the seven base units of SI from which all other SI units are defined.
Unit Name |
Unit Symbol |
Quantity |
Formal Definition |
Dimension Symbol |
|---|---|---|---|---|
metre |
$\text{m}$ |
length |
The distance travelled by light in a vacuum during a time interval of $^1/_{299,792,458}$ of a second. ($299,792,458\text{ms}^{-1}$ is the speed of light.) |
$\text{L}$ |
kilogram |
$\text{kg}$ |
mass |
The mass of the international prototype of the kilogram. |
$\text{M}$ |
second |
$\text{s}$ |
time |
The duration of $9,192,631,770$ periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom. |
$\text{T}$ |
ampere |
$\text{A}$ |
electric current |
The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to $2\times10^{-7}$ newton per meter length. |
$\text{I}$ |
kelvin |
$\text{K}$ |
thermodynamic temperature |
The fraction $^1/_{273.16}$ of the thermodynamic temperature of the triple point of water. |
$\Theta$ |
mole |
$\text{mol}$ |
amount of substance |
The amount of substance of a system which contains as many elementary entities as there are atoms of 0.012 kilogram of carbon 12. When a mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles or specified groups of such particles. |
$\text{N}$ |
candela |
$\text{cd}$ |
luminous intensity |
The luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency $540\times10^{12}$ hertz and that has a radiant intensity in that direction of $^1_{683}$ watt per steradian. |
$\text{J}$ |
The following table gives some commonly used SI units derived from the seven base units.
Note: This is not a comprehensive list.
Unit Name |
Unit Symbol |
Quantity |
Expressed in terms of other SI units |
Expressed in terms of SI base units |
|---|---|---|---|---|
radian |
$\text{rad}$ |
angle |
1 |
$\text{m}/\text{m}$ |
steradian |
$\text{sr}$ |
solid angle |
1 |
$\text{m}^2\text{m}^2$ |
Hertz |
$\text{Hz}$ |
frequency |
$\text{s}^{-1}$ |
|
newton |
$\text{N}$ |
force |
$\text{kg}\cdot\text{m}\cdot\text{s}^{-1}$ |
|
pascal |
$\text{Pa}$ |
pressure |
$\text{N}/\text{m}^2$ |
$\text{kg}\cdot\text{m}^{-1}\cdot\text{s}^{-2}$ |
joule |
$\text{J}$ |
energy |
$\text{N}\cdot\text{m}$ |
$\text{kg}\cdot\text{m}^2\cdot\text{s}^{-2}$ |
watt |
$\text{W}$ |
power |
$\text{J}/\text{s}$ |
$\text{kg}\cdot\text{m}^2\cdot\text{s}^{-3}$ |
coulomb |
$\text{C}$ |
electrical charge |
$\text{s}\cdot\text{A}$ |
|
volt |
$\text{V}$ |
voltage (electrical potential difference} |
$\text{W}/\text{A}$ |
$\text{kg}\cdot\text{m}^2\cdot\text{s}^{-3}\cdot\text{A}^{-1}$ |
farad |
$\text{F}$ |
electrical capacitance |
$\text{C}/\text{V}$ |
$\text{kg}^{-1}\cdot\text{m}^{-2}\cdot\text{s}^4\cdot\text{A}^2$ |
ohm |
$\Omega$ |
electrical resistance |
$\text{V}\text{A}$ |
$\text{kg}\cdot\text{m}^2\cdot\text{s}^{-3}\cdot\text{A}^{-2}$ |
siemens |
$\text{S}$ |
electrical conductance |
$\text{A}/\text{V}$ |
$\text{kg}^{-1}\cdot\text{m}^{-2}\cdot\text{s}^{3}\cdot\text{A}^{2}$ |
weber |
$\text{Wb}$ |
magnetic flux |
$\text{V}\cdot\text{s}$ |
$\text{kg}\cdot\text{m}^{2}\cdot\text{s}^{-2}\cdot\text{A}^{-1}$ |
tesla |
$\text{T}$ |
magnetic field strength |
$\text{Wb}/\text{m}^2$ |
$\text{kg}\cdot\text{s}^{-2}\cdot\text{A}^{-1}$ |
henry |
$\text{H}$ |
inductance |
$\text{Wb}/\text{A}$ |
$\text{kg}\cdot\text{m}^{2}\cdot\text{s}^{-2}\cdot\text{A}^{-2}$ |
degree Celcius |
$^{\circ}\text{C}$ |
temperature relative to $273.15\;K$ |
$\text{K}$ |
|
lumen |
$\text{lm}$ |
luminous flux |
$\text{cd}\cdot\text{sr}$ |
$\text{cd}$ |
lux |
$\text{lx}$ |
illuminance |
$\text{lm}/\text{m}^2$ |
$\text{m}^{-2}\text{cd}$ |