The 20 SI Prefixes

MultipleAmountPrefixSymbol
10241 million million-million-millionsyottaY
10211 thousand million-million-millionszettaZ
10181 million million-millionsexaE
10151 thousand million-millionspetaP
10121 million millionsteraT
1091 thousand millionsgigaG
1061 millionmegaM
1031 thousandkilok
1021 hundredhectoh
101tendecada
Sub-multipleFractionPrefixSymbol
10-11 tenthdecid
10-21 hundredthcentic
10-31 thousandthmillim
10-61 millionthmicroµ
10-91 thousandth of a millionthnanon
10-121 millionth of a millionthpicop
10-151 thousandth of a millionth of a millionthfemtof
10-181 millionth of a millionth of a millionthattoa
10-211 thousandth of a millionth of a millionth of a millionthzeptoz
10-241 millionth of a millionth of a millionth of a millionthyoctoy

All prefixes should be printed in upright type with no spaces or periods between the prefix and the unit symbol. i.e. kilometre = km, not k m, k.m or km

Prefixes should not be used on its own or combined into multiples compound prefixes. i.e. pg not µµg; 10 µg, not mcg.

The base unit “kg” already contains the prefix “k” for “kilo” (1000). Prefixes should be added to the physical quantity “gram”. i.e. mg not µkg; Mg, not kkg.

The prefix case should not be changed. Mg = 1 000 000 grams (1000 kg), not 1/1000 of a gram (1 mg).

The prefixes are not used with °C.

The prefix for “micro” is the Greek letter “mu”. In Windows driven computers this symbol can be achieved by holding down the “Alt” key and typing the number “230”. If this symbol is not available the most appropriate substitution is the letter “u” and not the abbreviation “mc.”

The grouping formed by a prefix symbol attached to a unit symbol constitutes a new inseparable unit symbol (forming a multiple or submultiple of the unit concerned) that can be raised to a positive or negative power and that can be combined with other unit symbols to form compound unit symbols.

Examples: 2.3 cm3 = 2.3 (cm)3 = 2.3 (10–2 m)3 = 2.3 x 10–6 m3
1 cm–1 = 1 (cm)–1 = 1 (10–2 m)–1 = 102 m–1 = 100 m–1
1 V/cm = (1 V)/(10–2 m) = 102 V/m = 100 V/m
5000 µs–1 = 5000 (µs)–1 = 5000 (10–6 s)–1 = 5 x 109 s–1

Similarly prefix names are also inseparable from the unit names to which they are attached. Thus, for example, millimetre, micropascal, and meganewton are single words.

Compound prefix symbols, that is, prefix symbols formed by the juxtaposition of two or more prefix symbols, are not permitted. This rule also applies to compound prefix names.

Prefix symbols can neither stand alone nor be attached to the number 1, the symbol for the unit one. Similarly, prefix names cannot be attached to the name of the unit one, that is, to the word “one.”

Prefix names and symbols are used with a number of non-SI units, but they are never used with the units of time: minute, min; hour, h; day, d. However astronomers use milliarcsecond, which they denote mas, and microarcsecond, µas, which they use as units for measuring very small angles.

Source: Bureau International des Poids et Mesures