üks kaheteistkümnendik süsinik-12 põhiolekus aatomi massist, võrdne aatommassi ühikuga (m)u=1.660 5402(10)×10−27 kg, 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 .