https://doi.org/10.1351/goldbook.K03391

The kilogram, symbol \(\text{kg}\), is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10^{−34} when expressed in the unit J s, which is equal to kg m^{2} s^{-1}, where the metre and the second are defined in terms of *c* and Δ*ν*_{Cs}.*Source: *

BIPM, The International System of Units, SI Brochure, 9^{th} ed. (2019), p. 131 (https://www.bipm.org/en/publications/si-brochure/)

BIPM, The International System of Units, SI Brochure, 9