Science Constants - Dalton Symbol: amu mu u Value: 1.66053873(13) × 10-27 kg 931.494013(37) MeV 1.49241778(12) × 10-10 J Category: - Physico-Chemical Constants Description: The atomic mass unit (or dalton) is defined as one twelfth of the mass of the most abundant isotope of carbon - 12C.   Prior to 1960, the atomic mass unit was defined in terms of the mass of a 16O isotope - 1 amu = 1.6599 x 10-27 kg.