\(\text{Ar X} = \dfrac{\text{massa 1 atom X}}{\frac{1}{12} \text{ massa 1 atom C-12}}\)
Jika unsur X memiliki beberapa isotop, maka rata-rata massa atom relatif unsur X (Ar X) adalah:
\(\text{Ar X} = \% \:.\: \text{Ar X}_1 + \% \:.\: \text{Ar X}_2 + \dotso\)