atomic mass constant

https://dx.doi.org/10.1351/goldbook.A00497
One twelfth of the mass of a carbon-12 atom in its nuclear and electronic ground state, m u = 1.660 5402 10 × 10 −27 kg. It is equal to the unified atomic mass unit
Source:
PAC, 1996, 68, 957 (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 962