Femto-

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Femto- (symbol f) is a prefix in the metric system denoting a factor of 10−15 or 0.000000000000001. Adopted by the 11th Conférence Générale des Poids et Mesures,[1] it was added in 1964 to the SI.[2] It is derived from the Danish word femten, meaning "fifteen".

Example of use:

  • a proton has a diameter of about 1.6 to 1.7 femtometres.

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.

Metric prefixes
Prefix Symbol 1000m 10n Decimal English word Since[n 1]
short scale long scale
yotta Y  10008  1024 1000000000000000000000000  septillion  quadrillion 1991
zetta Z  10007  1021 1000000000000000000000  sextillion  thousand trillion 1991
exa E  10006  1018 1000000000000000000  quintillion  trillion 1975
peta P  10005  1015 1000000000000000  quadrillion  thousand billion 1975
tera T  10004  1012 1000000000000  trillion  billion 1960
giga G  10003  109 1000000000  billion  thousand million 1960
mega M  10002  106 1000000             million 1960
kilo k  10001  103 1000             thousand 1795
hecto h  10002/3  102 100             hundred 1795
deca da  10001/3  101 10             ten 1795
 10000  100 1             one
deci d  1000−1/3  10−1 0.1             tenth 1795
centi c  1000−2/3   10−2 0.01             hundredth 1795
milli m  1000−1  10−3 0.001             thousandth 1795
micro µ  1000−2  10−6 0.000001             millionth 1960
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 six prefixes. The other dates relate to recognition by a resolution of the CGPM.

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