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 crosssection, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 × 107 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 

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
Quantity 
Name of Unit 
Symbol 
plane angle  radian  rad 
solid angle  steradian  sr 
Derived units
Quantity 
Name 
Symbol 
Equivalent to 
force  newton  N  kg.m/s^{2} 
pressure  pascal  Pa  N/m^{2} 
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/m^{2} 
frequency  hertz  Hz  s1 
luminous flux  lumen  lm  cd.sr 
illuminance  lux  lx  lm/m^{2} 
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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 nonSI 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).
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
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 000103hectoh100102decada10101decid0.1101centic0.01102millim0.001103microµ0.000 001106