https://doi.org/10.1351/goldbook.K03391
The kilogram, symbol 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 15×1034 when expressed in the unit J s, which is equal to kgm2s1, where the metre and the second are defined in terms of c and ΔνCs.
Source:
BIPM, The International System of Units, SI Brochure, 9th ed. (2019), p. 131 [Terms] [Book]