We use unit prefixes to change the size of other units and they also make the calculation easy and memorable. Similar to scientific notation, you can increase or decrease the size of unit using unit prefixes. Once the standard units are defined you can derive smaller or larger forms of those units using unit prefixes. The unit prefixes chart (table) below shows the prefixes that can be used with other units.

PrefixPrefix SymbolPower of 10Examples of Meter
yottaY${{10}^{24}}$1Ym = 1 yottameter = ${{10}^{24}}$m
zettaZ${{10}^{21}}$1Zm = 1 zettameter = ${{10}^{21}}$m
exaE${{10}^{18}}$1Em = 1 exameter = ${{10}^{18}}$m
petaP${{10}^{15}}$1Pm = 1 petameter = ${{10}^{15}}$m
teraT${{10}^{12}}$1Tm = 1 terameter = ${{10}^{12}}$m
gigaG${{10}^{9}}$1Gm = 1 gigameter = ${{10}^{9}}$m
megaM${{10}^{6}}$1Mm = 1 megameter = ${{10}^{6}}$m
kilok${{10}^{3}}$1km = 1 kilometer = ${{10}^{3}}$m
hectoh${{10}^{2}}$1hm = 1 hectometer = ${{10}^{2}}$m
decada${{10}^{1}}$1dam = 1 decameter = ${{10}^{1}}$m
--${{10}^{0}}$-
decid${{10}^{-1}}$1dm = 1 decimeter = ${{10}^{-1}}$m
centic${{10}^{-2}}$1cm = 1 centimeter = ${{10}^{-2}}$m
millim${{10}^{-3}}$1mm = 1 millimeter = ${{10}^{-3}}$m
micro$\mu $${{10}^{-6}}$1$\mu $m = 1 micrometer = ${{10}^{-6}}$m
nanon${{10}^{-9}}$1nm = 1 nanometer = ${{10}^{-9}}$m
picop${{10}^{-12}}$1pm = 1 picometer = ${{10}^{-12}}$m
femtof${{10}^{-15}}$1fm = 1 femtometer = ${{10}^{-15}}$m
atto$a$${{10}^{-18}}$1$a$m = 1 attometer = ${{10}^{-18}}$m
zeptoz${{10}^{-21}}$1zm = 1 zeptometer = ${{10}^{-21}}$m
yoctoy${{10}^{-24}}$1ym = 1 yoctometer = ${{10}^{-24}}$m

The standard units we use in Physics and all over the world are the SI units, and these units are based on the metric system of units. We generally use these prefixes with units from the metric system and they are also called metric unit prefixes. But these prefixes are not only used with the SI units but can also be used with any other unit.

Note that gram($\text{g}$) is not the SI unit and we use these prefixes with gram not kilogram even if kilogram (it already has the prefix 'kilo') is the SI unit. Some other examples are millimeter($\text{mm}$), kilometer($\text{km}$), kilowatt($\text{kW}$), mililiter($\text{ml}$), centiliter($\text{cl}$), megavolt($\text{MV}$), megapixel etc.

In Physics we often use the prefixes shown in the unit prefixes list in above table to define new units. Sometimes we also have specific name for unit with prefix, for example $1\mu m$ = 1 micron. Here $\mu$ is the Greek letter pronounced as mu.

Some examples of mass and time are given in the table below.

MassTime
1 microgram ($\mu$g) = $10^{-9}$kg1 nanosecond (ns) = ${{10}^{-9}}\text{s}$
1 miligram (mg) = $10^{-6}$kg1 microsecond ($\mu $s) = ${{10}^{-6}}$s
1 gram (g) = $10^{-3}$kg1 millisecond (ms) = ${{10}^{-3}}$s

These prefixes represent the multiples of 10 or 1/10 (power of 10). We always prefer to use these prefixes in calculations as we always need to deal with very large and very small numbers, and these prefixes make it easy to remember and memorable. To be able to deal with very small and large numbers we have a different notation of number called scientific notation.

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