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Femto- (symbol f) is a unit prefix in the metric system denoting a factor of 10−15 or Script error: No such module "Gaps".. 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 femten, meaning "fifteen".
Example of use:
- a proton has a diameter of about 1.6 to 1.7 femtometres.