Atoms are far too light (~10⁻²⁷ kg) to weigh on any balance, so chemistry uses a relative mass scale instead of absolute mass. IUPAC fixed the carbon-12 atom as the standard: one atomic mass unit (1 amu or 1 u) is defined as one-twelfth of the mass of a carbon-12 atom in its ground state (1 u ≈ 1.6605 × 10⁻²⁷ kg).

Relative atomic mass (Aᵣ) of an element = (average mass of its atom) ÷ (1 u). Because most elements are naturally a mixture of isotopes (same number of protons, different numbers of neutrons, hence different mass), the atomic mass we use is an average atomic mass: the abundance-weighted average across all naturally occurring isotopes.