The femtometre (American spelling femtometer) symbol fm[1][2][3] derived from the Danish and Norwegian word femten 'fifteen', Ancient Greek: μέτρον, romanized: metrοn, lit.'unit of measurement') is a unit of length in the International System of Units (SI) equal to 10−15 metres, which means a quadrillionth of one metre. This distance is sometimes called a fermi and was so named in honour of Italian-American physicist Enrico Fermi, as it is a typical length-scale of nuclear physics.

Unit systemSI
Unit oflength
1 fm in ...... is equal to ...
   SI base units   1×10−15 m
   Natural units   6.1877×1019 P
1.8897×10−5 a0
   imperial/US units   3.9370×10−14 in
The helium atom and perspective magnitudes

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