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# Electricity > SI Prefixes

An SI prefix describes a symbol that is placed before an [SI unit] to form a decimal multiple or submultiple value. A simple example used on a day to day basis is the kilogram (kg). It is the only SI unit to have an SI prefix in place as standard. The small 'k' represents a thousand (see chart below) and therefore 'kg' means a 'thousand grams.'

As with SI units, prefixes are case sensitive. The differences between 1mW and 1MW are immense. 1mW=0.001W whereas 1MW=1000000W.

The most common prefixes used in schools are in bold type.

10n Prefix Symbol Name Decimal equivalent in SI
1024 yotta Y Septillion 1 000 000 000 000 000 000 000 000
1021 zetta Z Sextillion 1 000 000 000 000 000 000 000
1018 exa E Quintillion 1 000 000 000 000 000 000
1015 peta P Quadrillion 1 000 000 000 000 000
1012 tera T Trillion 1 000 000 000 000
109 giga G Billion 1 000 000 000
106 mega M Million 1 000 000
103 kilo k Thousand 1 000
102 hecto h Hundred 100
101 deca da Ten 10
100

(none)

(none)

One 1
10−1 deci d Tenth 0.1
10−2 centi c Hundredth 0.01
10−3 milli m Thousandth 0.001
10−6 micro µ (u) Millionth 0.000 001
10−9 nano n Billionth 0.000 000 001
10−12 pico p Trillionth 0.000 000 000 001
10−15 femto f Quadrillionth 0.000 000 000 000 001
10−18 atto a Quintillionth 0.000 000 000 000 000 001
10−21 zepto z Sextillionth 0.000 000 000 000 000 000 001
10−24 yocto y Septillionth 0.000 000 000 000 000 000 000 001