Average Atomic Mass Calculator

Weighted average atomic mass from isotope masses and percent abundances, or the abundances from the average.

Average atomic mass = Σ(isotope mass × fractional abundance). Chlorine: 34.969 × 0.7576 + 36.966 × 0.2424 = 35.45 u.

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How to use it

Enter each isotope's mass (in u) and its abundance in percent, or pick a preset element.
Add rows for elements with more than two isotopes.
To find abundances, use the reverse section with the average and two isotope masses.

Ā = Σ mᵢ × (abundanceᵢ / 100) | Two isotopes: x₁ = (Ā − m₂) / (m₁ − m₂)

Input: ³⁵Cl 34.96885 u at 75.76%, ³⁷Cl 36.96590 u at 24.24%

Result: 35.45 u

Matches the periodic table value because ³⁵Cl is three times as common.

Multiply each isotope's mass by its abundance as a decimal (75.76% is 0.7576) and add the results.

For two isotopes, let x be the fraction of isotope 1: x = (average − m₂) ÷ (m₁ − m₂). Copper's 63.546 average gives 69.15% copper-63.

It is a weighted average of several isotopes, and each isotope's mass differs slightly from its mass number because of nuclear binding energy.

Numerically yes: an average atomic mass of 35.45 u means a molar mass of 35.45 g/mol.

Natural abundances vary slightly between sources; IUPAC gives intervals for some elements.