Fermi (unit)

For examples of things measuring between one and ten femtometres, see 1 femtometre.

The femtometre (American spelling femtometer, symbol fm[1][2][3]) (Danish: femten, "fifteen"; Ancient Greek: μέτρον, metrοn, "unit of measurement") is an SI unit of length equal to 10−15 metres. This distance can also be called fermi and was so named in honour of physicist Enrico Fermi, as it is a typical length-scale of nuclear physics.

Definition and equivalents

1 femtometre = 1.0 x 10−15 metres = 1 fermi = 0.001 picometre = 1000 attometres

1,000,000 femtometres = 1 nanometre.

For example, the diameter of a gold nucleus is approximately 8.45 femtometres.


The femtometre was adopted by the 11th Conférence Générale des Poids et Mesures, and added to SI in 1964.

The fermi is named after the Italian physicist Enrico Fermi (1901–1954), one of the founders of nuclear physics. The term was coined by Robert Hofstadter in a 1956 paper published in Reviews of Modern Physics entitled "Electron Scattering and Nuclear Structure".[4] The term is widely used by nuclear and particle physicists. When Hofstadter was awarded the 1961 Nobel Prize in Physics, it subsequently appears in the text of his 1961 Nobel Lecture, "The electron-scattering method and its application to the structure of nuclei and nucleons" (December 11, 1961).[5]


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