Metric prefixes

Explanation of prefixes

A metric prefix is a convenient way of expressing mulitiples and subdivisions (larger and smaller) of any defined unit. In metric that means multiplying or dividing by 10, 100, 1000, etc. It consists of a partial word like “kilo” or “milli”.

For example, the prefix “kilo” means “times a thousand” or “one thousand of” the units in question. Sokilometre” means one thousand metres, and “kilogram” means one thousand grams.

Likewise, they can mean a fraction of a unit rather than a multiple. For example, the prefix “milli” means “one thousandth of”. So a millimetre is one thousandth of a metre. Put another way, there are one thousand millimetres in a metre.

These prefixes can be applied to any fundamental metric unit like the metre for distance, the gram for mass or weight, the watt for power, and so on.

Why use prefixes instead of distinct names?

Traditional measurement units that were used before the metric system had their own distinct names like stone, pound and ounce for weight or yard, foot and inch for distance. Some people criticise the metric system because of the long multisyllable names it gives rise to as a result of the prefixes. They do not exactly “trip off the tongue” or lend themselves to poetry.

This minor inconvenience has to be weighed against the more important advantages which are:

  1. The names are self explanatory. The two part name construction exposes the fundamental unit to which the named unit is referenced and by a factor which is encoded by the name itself.
  2. The names are easier to learn and remember. Only the set of prefixes need to be learned which can then be applied to any type of measurement. If distinct names were used there would be a plethora of names to learn for each type of measurement.
  3. The prefixes have a structure. The multiples and subdivisions are related by a consistent structure that is easy to understand. Most of the prefixes are related by multiples of 1000. A few which are seldom used nowadays are related by the compatible numbers 10 and 100.
  4. Prefixes are understood internationally. A standard set of symbols correspond to the metric prefixes (e.g. k- for kilo- and m- for milli-). They are recognised internationally and are independent of language.

SI Prefixes

Common prefixes

The table below includes the most commonly encountered prefixes based on multiples or subdivisions of 1000.

Prefix Symbol Multiple Examples
kilo k 1000 kilometre (km),
kilogram (kg),
kilowatt (kW)
mega M 1 000 000
(1 million)
megawatt (MW) radio transmitter output,
megahertz (MHz)
FM radio frequencies
giga G 1 000 000 000
(1 billion)
gigawatt (GW) power station output,
gigahertz (GHz)
computer processor speed
Prefix Symbol Fraction Examples
milli m 1/1000 millimetre (mm),
millilitre (ml or mL),
milligram (mg) drug doses
micro μ 1/1 000 000
(0.000 001)
micrometre (µm)
precision engineeering,
microgram (µg) drug doses
nano n 1/1 000 000
(0.000 000 001)
nanometre (nm)

Note that the examples merely illustrate typical usage and are not exhaustive.

The less commonly used prefixes with multiples and subdivisions between 1/1000 and 1000.

Prefix Symbol Multiple Examples
deca da 10 Rarely used
hecto h 100 hectolitre (hl or hL) – used to measure wine production

‘hectare’ (ha) is in fact a contraction of ‘hecto are’ where the ‘are’ is an old unit of area equal to 100 square metres (10 m x 10 m).

Hence 1 ha = 100 m x 100 m, which is 10 000 m2

Prefix Symbol Fraction Examples
deci d 1/10
decilitre (dl or dL) sometimes used for blood sugar levels (mg/dL)
centi c 1/100
centimetre (cm),
centilitre (cl or cL) wine bottles

Complete list

The table below is the complete list of all SI prefixes and their factors using exponential (scientific) notation.

Name Symbol Value Name Symbol Value
yotta Y 1024 yocto y 10-24
zetta Z 1021 zepto z 10-21
exa E 1018 atto a 10-18
peta P 1015 femto f 10-15
tera T 1012 pico p 10-12
giga G 109 nano n 10-9
mega M 106 micro μ 10-6
kilo k 103 milli m 10-3
hecto h 102 centi c 10-2
deca d 101 deci d 10-1