The yoctometre (International spelling as used by the International Bureau of Weights and Measures; SI symbol: ym) or yoctometer (American spelling) is an SI measure of length that is equal to 10−24 (yocto) of a metre. It is the smallest named subdivision of the metre in the SI base unit of length.
In terms of relative length of units, a yoctometre ranks as follows: Planck length << yoctometre << zeptometre. A yoctometre is 1/1000 of a zeptometre (1 zeptometre (zm) = 1,000 yoctometres). The Planck length is about 1.6 × 10−11 ym.