SI prefix

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SI prefixes for SI units are:

(Sub)multiplePrefixSymbolName (US and Canada)Name (Europe)
1024yottaYSeptillionQuadrillion
1021zettaZSextillionTrilliard
1018exaEQuintillionTrillion
1015petaPQuadrillionBilliard
1012teraTTrillionBillion
109gigaGBillionMilliard
106megaMMillionMillion
103kilokThousandThousand
102hectohHundredHundred
101decadaTenTen
100isoiUnitUnit
10-1decidTenthTenth
10-2centicHundredthHundredth
10-3millimThousandthThousandth
10-6microμMillionthMillionth
10-9nanonBillionthMilliardth
10-12picopTrillionthBillionth
10-15femtofQuadrillionthBilliardth
10-18attoaQuintillionthTrillionth
10-21zeptozSextillionthTrilliardth
10-24yoctoySeptillionthQuadrillionth

Examples:

  • 5 dm = 5 x 10-2m = 5 x 0.01 m = 0.05 m
  • 3 MW = 3 x 106W = 3 x 1 000 000 W = 3 000 000 W

Note that when used with non-SI units, prefices occasionally have other values. For instance, when referring to units of storage like bytes, kilo means 210=1024 rather than 1000.

Prefixes where the exponent is divisible by three are recommended. Hence '100 metres' rather than 'one hectometre'. Notable exceptions include centimetre, hectare (hecto-are), centilitre (being equivalent to 1m3), and 1 dm3 (equivalent to one litre, sometimes referred to as a 'dumphrey').


In Britain and Ireland the names that end in "-iard" and "-iardth" are not used. Instead for example 109 is referred to as a "thousand million", though the U.S. naming is also sometimes used.