atomic mass constant

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:

CODATA Bull. 1986, 63, 1

PAC, 1996, 68, 957 (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 962