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pre. In the International System of Units and other metric systems of units, multiplying the unit to which it is attached by 10−15. Symbol: f.


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

Examples of use:

  • The HIV-1 virus weighs about 1 x 10 kg or 1 fg. Orders of magnitude (mass)
  • a proton has a diameter of about 1.6 to 1.7 femtometres.
  • More examples available.

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.