The modernized metric system is known as the International System of Units, usually abbreviated to SI, from the French, "Système internationald'unités".

Base on the number 10, SI is today the world's most widely used system of measurement in science, industry and commerce.

Units

Each SI unit has a specific symbol, which is the same in all languages. For example, in Irish the word for kilogram is 'cileagram', but the symbol is kg.

Base units

SI definitions are founded on seven base units, as set out in the table below.

Quantity

Unit

Symbol

Definition

Length metre m The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.
Mass kilogram kg The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.
Time second s The second is 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.
Electric current ampere A 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 × 10-7 newton per metre of length.
Thermodynamic temperature kelvin K The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.
Amount of substance mole mol
  1. The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12.
  2. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.
Luminous intensity candela cd The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.


Supplementary units

In addition, there are two supplementary units.
 

Quantity

Name of Unit

Symbol

plane angle radian rad
solid angle steradian sr

Derived units

These are derived with base and/or supplementary units. They are expressed by using the SI symbols with multiplication and/or division signs.

 

Quantity

Name

Symbol

Equivalent to

force newton N kg.m/s2
pressure pascal Pa N/m2
work, energy, quantity of heat joule J N.m
power, heat flow rate watt W J/s
quantity of electricity coulomb C A.s
electric potential volt V W/A
electric resistance ohm Ω V/A
electric capacitance farad F C/V
magnetic flux weber Wb V.s
inductance henry H Wb/A
magnetic flux density tesla T Wb/m2
frequency hertz Hz s-1
luminous flux lumen lm cd.sr
illuminance lux lx lm/m2

Prefixes

Decimal multiples and submultiples of SI units can be expressed by adding prefixes to the unit names. The prefix is directly attached to the unit name to form a new unit name, which is written as one word, e.g. kilometre; millimetre.

The prefix symbol is directly attached to the unit symbol to form a new unit symbol: km for kilometre; mm for millimetre.
 

Prefix

Symbol

Means multiply by

Or by

mega M 1 000 000 106
kilo k 1 000 103
hecto h 100 102
deca da 10 101
deci d 0.1 10-1
centi c 0.01 10-2
milli m 0.001 10-3
micro µ 0.000 001 10-6

SI Exceptions

Certain units have been used for many centuries and retain historical, cultural and practical importance.

Consequently, these units are recognized for use with SI.

Quantity

Name

Symbol

Value in SI Units

time minute min 1 min = 60 s
  hour h 1 h = 3 600 s
  day d 1 d = 86 400
  year a  
plane angle degree º 1 º = (∏/180) rad
  minute ' 1 ' = (∏/10 800) rad
  second '' 1 '' = (∏/648 000) rad
area hectare ha 1 ha = 1 hm2 = 10 000 m2
volume litre L or l 1 L = 1dm3
mass tonne t 1 t = 1 000 kg = 1 Mg