SI 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

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How Do I Write SI Units?

The basic rules for writing symbols

The symbols are always printed in upright type, irrespective of the typeface used in the text: m N s

Symbols are written in lower case (not capitals), except when the unit name is derived froma proper name: m for meter, s for second, but N for newton, A for ampere. The exeception is L for litre

Prefix symbols are printed in upright type without spacing between the prefix symbol and the unit symbol; kg for kilogram, km for kilometre

Symbols are never pluralized: 1 g, 45 g (not 45gs)

Names and symbols should not be mixed: N.M or newton metre, but not N metre or newton m

Never use a full stop after a symbol, except when it occurs at the end of a sentence

Always use a full space between the numeral and the symbol: 45 g (not 45g). Exception: When the first character of a symbol (for non-SI unit) is not a letter, no space is left: 32°C (not 32° C or 32 °C), 75° 12' 45" (not 75 ° 12 ' 45 ")

Symbols should be used in conjunction with numerals instead of writing out the unit names; when no numerals are involved unit names should be written out: Area of carpet is 16 m2 (not 16 square metres). Carpet is sold by square metre (not m2)

The product of two or more units in symbolic form may be indicated by a space or a dot: m s or m.s - metre second (not ms which would mean millisecond).

What are the Exceptions to the SI Units?

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

timeminutemin1 min = 60 s
hourh1 h = 3 600 s
dayd1 d = 86 400
yeara
plane angledegreeº1 º = (∏/180) rad
minute'1 ' = (∏/10 800) rad
second''1 '' = (∏/648 000) radareahectareha1 ha = 1 hm2 = 10 000 m2volumelitreL or l1 L = 1dm3masstonnet1 t = 1 000 kg = 1 Mg

What are Prefixes?

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

megaM1 000 000106kilok1 000103hectoh100102decada10101decid0.110-1centic0.0110-2millim0.00110-3microµ0.000 00110-6