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 the 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
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. The units in metric system are the SI units. We generally use these prefixes with units from metric system and they are sometimes called metric unit prefixes. But these are not only used with the SI units. We can use these prefixes 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 below table.

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).

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