Average Atomic Mass Calculator

Find an element’s weighted average atomic mass using isotope masses and their relative abundances.

How the average atomic mass calculator works

Average atomic mass = Σ(isotope mass × relative abundance) ÷ Σ(relative abundance). Abundances may be entered as percentages; this calculator normalizes the entered values if their total is not exactly 100%.

Frequently asked questions

Do isotope abundances need to total exactly 100%?

They should represent the isotope distribution. This calculator normalizes the values you enter, but check that the abundances come from a consistent source.

What unit is atomic mass reported in?

The result is expressed in unified atomic mass units (u), often called atomic mass units.

Results are estimates for general information only. They are not a substitute for professional advice where applicable. Found a mistake? Report an error.