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Average Atomic Mass from Isotopic Abundances Calculator

M_avg = Σ (abundance_i · mass_i) — over up to three isotopes.

Compute the weighted-average atomic mass of an element from the isotopic masses (atomic mass units) and natural-abundance percentages of up to three isotopes. The abundances should sum to ≈ 100 %; the calculator reports both the average atomic mass and the abundance-sum check value (any non-100 indicates a missing isotope).

Dr. Helena Ortiz By Dr. Helena Ortiz
Published Last reviewed 1 min read

Inputs

amu
%
amu
%
amu
%

Results

Enter values and click Calculate to see results.
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How to use this calculator

  1. Fill in the inputs above using the units you already have.
  2. Values update automatically as you type — no submit button needed.
  3. Hover any result row for the underlying formula and intermediate values.

Formula

M_avg = Σᵢ (aᵢ · mᵢ), where aᵢ is the fractional abundance.

In depth

Compute the weighted-average atomic mass of an element from the isotopic masses (atomic mass units) and natural-abundance percentages of up to three isotopes. The abundances should sum to ≈ 100 %; the calculator reports both the average atomic mass and the abundance-sum check value (any non-100 indicates a missing isotope).