The International System of Units, SI

The International System of Units, denoted SI in all languages, was adopted by the 11th International Conference on Weights and Measures. The SI is a coherent system of units with respect to the International System of Quantities.

The SI comprises

⎯ base units, and

⎯ derived units

that together form the coherent system of SI units.

SI base units
Base quantity Base unit
Name Symbol
length metre m
mass kilogram kg
time second s
electric current ampere A
thermodynamic temperature kelvin K
amount of substance mole mol
luminous intensity candela cd

SI base units
Derived quantity SI derived unit
Symbol Expressed in
SI base
units and
SI derived
plane angle radian rad rad = m/m = 1
solid angle steradian sr sr = m2/m2 = 1
frequency hertz Hz Hz = s-1
force newton N N = kg·m/s2
pressure, stress pascal Pa  Pa = N/m2
energy joule J J = N m
power watt W W= J/s
electric charge coulomb C C = A · s
electric potential difference volt V V = W/A
capacitance farad F F = C/V
electric resistance ohm Ω = V/A
electric conductance siemens S = Ω-1
magnetic flux weber Wb Wb = V · s
magnetic flux density tesla T T = Wb / m2
inductance henry H H = Wb/A
Celsius temperature degree Celsius °C °C = K
luminous flux lumen lm lm = cd · sr
illuminance lux lx lx = lm/m2



  • ISO 80000-1 : 2009, Quantities and Units —— Part 1: General