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Femto (symbol f) is a unit prefix in the metric system denoting a factor of 10-15. It was added to the International System of Units (SI) in 1964.[1] It is derived from the Danish word femten, meaning "fifteen", which has a vaguely similar spelling to fermi, which was introduced earlier to mean a femtometer.

Examples of use:

The femtometre shares the unit symbol (fm) with the older non-SI unit fermi, to which it is equivalent. The fermi, named in honour of Enrico Fermi, is often encountered in nuclear physics.

SI prefixes
Prefix Base 10 Decimal English word Adoption[nb 1] Etymology
Name Symbol Short scale Long scale Language Derived word
yotta Y  1024 1000000000000000000000000  septillion  quadrillion 1991 Greek eight[nb 2]
zetta Z  1021 1000000000000000000000  sextillion  trilliard 1991 Latin seven[nb 2]
exa E  1018 1000000000000000000  quintillion  trillion 1975 Greek six
peta P  1015 1000000000000000  quadrillion  billiard 1975 Greek five[nb 2]
tera T  1012 1000000000000  trillion  billion 1960 Greek four[nb 2], monster
giga G  109 1000000000  billion  milliard 1960 Greek giant
mega M  106 1000000  million 1873 Greek great
kilo k  103 1000  thousand 1795 Greek thousand
hecto h  102 100  hundred 1795 Greek hundred
deca da  101 10  ten 1795 Greek ten
 100 1  one -
deci d  10-1 0.1  tenth 1795 Latin ten
centi c  10-2 0.01  hundredth 1795 Latin hundred
milli m  10-3 0.001  thousandth 1795 Latin thousand
micro ?  10-6 0.000001  millionth 1873 Greek small
nano n  10-9 0.000000001  billionth  milliardth 1960 Greek dwarf
pico p  10-12 0.000000000001  trillionth  billionth 1960 Spanish peak
femto f  10-15 0.000000000000001  quadrillionth  billiardth 1964 Danish fifteen, Fermi[nb 3]
atto a  10-18 0.000000000000000001  quintillionth  trillionth 1964 Danish eighteen
zepto z  10-21 0.000000000000000000001  sextillionth  trilliardth 1991 Latin seven[nb 2]
yocto y  10-24  0.000000000000000000000001  septillionth  quadrillionth 1991 Greek eight[nb 2]
  1. ^ Prefixes adopted before 1960 already existed before SI. The introduction of the CGS system was in 1873.
  2. ^ a b c d e f Part of the beginning of the prefix was modified from the word it was derived from, ex: "peta" (prefix) vs "penta" (derived word).
  3. ^ The fermi was introduced earlier with the same symbol "fm", in which then the "f" became a prefix. The Danish word is used since it is vaguely spelled similar to fermi.


  1. ^ "Resolution 8 of the 12th CGPM". International Bureau of Weights and Measures. 1964. Retrieved .

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