The Systeme International (SI) is the worldwide metric system. It allows the sizes of units to be made bigger or smaller by the use of appropriate prefixes.

For example, the electrical unit of a watt is not a big unit even in terms of ordinary household use, so it is generally used in terms of 1000 watts at a time. The prefix for 1000 is kilo so we use kilowatts [kW] as our unit of measurement. For makers of electricity, or bigger users such as industry, it is common to use megawatts [MW] or even gigawatts [GW].

yotta **Y** 1 000 000 000 000 000 000 000 000 10^24

zetta **Z** 1 000 000 000 000 000 000 000 10^21

exa **E** 1 000 000 000 000 000 000 10^18

peta **P **1 000 000 000 000 000 10^15

tera **T** 1 000 000 000 000 10^12

giga **G** 1 000 000 000 a billion

mega **M** 1 000 000 a million

kilo **k** 1 000 a thousand

hecto **h** 100

deca **da** 10

1

deci **d** 0.1

centi **c** 0.01

milli **m** 0.001 a thousandth

micro **µ** 0.000 001 a millionth

nano **n** 0.000 000 001 a thousand millionth

pico **p **0.000 000 000 001 10^-12

femto **f** 0.000 000 000 000 001 10^-15

atto **a** 0.000 000 000 000 000 001 10^-18

zepto **z** 0.000 000 000 000 000 000 001 10^-21

yocto **y** 0.000 000 000 000 000 000 000 001 10^-24

[µ] the symbol used for micro is the Greek letter mu.

Nearly all of the S I prefixes are multiples or sub-multiples of 1000. However, these are inconvenient for many purposes and so hecto, deca, deci, and centi are also used. Deca also appears as deka [da] or [dk] in the USA and Continental Europe.