fm
femtometer
Lengthexact by definition
The femtometre is 10⁻¹⁵ of a metre, an exact SI prefix on the base unit. It is the natural ruler of the atomic nucleus: a proton has a charge radius near 0.84 fm, and a uranium nucleus is roughly 7 fm across. Because it is a decimal prefix on the metre, the factor is exact by definition.
| 1 fm | 1.0000000e-15 m |
Where the unit came from
Nuclear physicists call the same length a fermi, after Enrico Fermi, and the two names are numerically identical. The coincidence that "fermi" and "femtometre" share an abbreviation is convenient rather than planned.
Solvers that use length