# Femto-

Femto- (symbol f) is a unit prefix in the metric system denoting a factor of 10−15 or 0.000000000000001. Adopted by the 11th General Conference on Weights and Measures,[1] it was added in 1964 to the SI.[2] It is derived from the Danish word femten, meaning "fifteen".

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.

Prefix 1000m 10n Decimal English word Adoption[nb 1]
Name Symbol Short scale Long scale
yotta Y  10008  1024 1000000000000000000000000  septillion  quadrillion 1991
zetta Z  10007  1021 1000000000000000000000  sextillion  thousand trillion or trilliard 1991
exa E  10006  1018 1000000000000000000  quintillion  trillion 1975
peta P  10005  1015 1000000000000000  quadrillion  thousand billion or billiard 1975
tera T  10004  1012 1000000000000  trillion  billion 1960
giga G  10003  109 1000000000  billion  thousand million or milliard 1960
mega M  10002  106 1000000             million 1960 (1873)
kilo k  10001  103 1000             thousand 1960 (1795)
hecto h  10002/3  102 100             hundred 1960 (1795)
deca da  10001/3  101 10             ten 1960 (1795)
10000  100 1             one
deci d  1000−1/3  10−1 0.1             tenth 1960 (1795)
centi c  1000−2/3   10−2 0.01             hundredth 1960 (1795)
milli m  1000−1  10−3 0.001             thousandth 1960 (1795)
micro μ  1000−2  10−6 0.000001             millionth 1960 (1873)
nano n  1000−3  10−9 0.000000001  billionth  thousand millionth 1960
pico p  1000−4  10−12 0.000000000001  trillionth  billionth 1960
femto f  1000−5  10−15 0.000000000000001  quadrillionth  thousand billionth 1964
atto a  1000−6  10−18 0.000000000000000001  quintillionth  trillionth 1964
zepto z  1000−7  10−21 0.000000000000000000001  sextillionth  thousand trillionth 1991
yocto y  1000−8  10−24  0.000000000000000000000001  septillionth  quadrillionth  1991
1. The metric system was introduced in 1795 with several metric prefixes, of which, however, only six were adopted as SI prefixes by the 11th CGPM conference in 1960, whereas myria (104) as well as double and demi were not adopted. In 1873, micro and mega were recommended by the British Association for the Advancement of Science. The other dates relate to recognition by a resolution of the CGPM.