11.1.12—The unified atomic mass unit (u) as a unit of mass

One unified atomic mass unit, u, is defined as one twelfth of the mass of a neutral carbon-12 atom, approximately 1.66×10⁻²⁷ kg.

Use u for comparing nuclear and particle masses, then convert to kilograms when equations require SI units.

A nuclide with mass number about 4 has mass roughly 4 u, before accounting for binding-energy mass defect.

u is a unit of mass, not atomic number or energy; mass number is an integer count, not an exact mass in u.