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.
SI definitions are founded on seven base units, as set out in the table below.
|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.
|The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.
|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.
|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.
|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
|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.
Name of Unit
|work, energy, quantity of heat
|power, heat flow rate
|quantity of electricity
|magnetic flux density
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).
Certain units have been used for many centuries and retain historical, cultural and practical importance.
Consequently, these units are recognized for use with SI.
Value in SI Units
timeminutemin1 min = 60 s
hourh1 h = 3 600 s
dayd1 d = 86 400
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
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.
Means multiply by
megaM1 000 000106kilok1 000103hectoh100102decada10101decid0.110-1centic0.0110-2millim0.00110-3microµ0.000 00110-6