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SI base units

SI stands for International System of units. The SI is the most common system of measurement used widely worldwide. It is also the latest form of the metric system of units and is based on 7 basic units, the so called SI base units and a set of standard prefixes based on the power of 10. The 7 base units of the International Systemare shown in the following table:

SI prefixes

The following is the list of prefixes used together with the base units and the units derived from them to form multiples and subdivision of the units.

Prefixes for unit multiples

These prefixes can be added to a unit to create a multiple of that unit (see the example column)
Prefix name Symbol Factor Example
deca da 101 dam, decaliter
hecto h 102 hm, hectometer
kilo k 103 km, kilometer
mega M 106 Mb, Megabyte
giga G 109 Gb, Gigabyte
tera T 1012 Tb, Terabyte
peta P 1015 Pt, Petabyte
exa E 1018 Eb, Exabyte
zeta Z 1021 Zb, Zetabyte
yotta Y 1024  Yb, Yottabyte

Prefixes for unit subdivisions

These prefixes can be added to a unit to create a subdivision of that unit (see the example column)
Prefix name Symbol Factor Example
deci d 10-1 dm, decimeter
centi c 10-2 cm, centimeter
milli m 10-3 mm, millimeter
micro µ 10-6 µm, micrometer
nano n 10-9 nm, nanometer
pico p 10-12 pm, picometer
fempto f 10-15 fm, femptometer
atto a 10-18  am, attometer
zepto z 10-21  zm, zeptometer
yocto y 10-24  ym, yoctometer

Non-SI prefixes (discontinued)

These prefixes can be added to a unit to create a subdivision of that unit (see the example column)
Prefix name Symbol Factor Example
hebdo H 107 Hm, hebdometer
lacta L 105 Lm, lactameter
myria my 104 mym, myriameter
dimi dm 10-4 dmm, dimimeter
micri mc 10-14 mcm, micrimeter

Binary prefixes for the power of two (used for measuring digital information)

These prefixes can be added to a unit to create a subdivision of that unit (see the example column)
Prefix name Symbol Factor Example
kibi Ki 210 KiB, kibibyte
mebi Mi 220 MiB, mebibyte
gibi Gi 230 GiB, gigibyte
tebi Ti 240 TiB, tebibyte
pebi Pi 250 PiB, pebibyte
exbi Ei 260 EiB, exbibyte
zebi Zi 270 ZiB, zebibyte
yobi Yi 280 YiB, yobibyte


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