Average Atomic Mass Calculator
Compute the weighted average atomic mass of an element from two isotopes’ masses and their percent abundances.
How it works
Average mass = (m₁ × a₁ + m₂ × a₂) ÷ (a₁ + a₂) · masses in u, abundances in %
The atomic mass on the periodic table is a weighted average over an element’s naturally occurring isotopes, not the mass of any single atom. Each isotope’s mass is weighted by its fractional abundance and the contributions are summed. Dividing by the total abundance normalises the result, so it stays correct even if the two abundances you enter do not add up to exactly 100%.
Worked example
Chlorine has ³⁵Cl (34.969 u, 75.77%) and ³⁷Cl (36.966 u, 24.23%). Average = (34.969 × 75.77 + 36.966 × 24.23) ÷ 100 = 35.45 u, matching the periodic-table value.
Sources: Relative atomic mass — Wikipedia
Frequently asked questions
Why isn’t the atomic mass a whole number?
Because it averages isotopes of different masses in their natural proportions. Chlorine’s 35.45 u sits between ³⁵Cl and ³⁷Cl, weighted toward the more abundant ³⁵Cl.
What if my abundances don’t total 100%?
The calculator divides by the actual sum of the abundances, so it normalises them automatically. For an accurate result, still use published abundances that add to 100%.
Can I use fractional abundances instead of percentages?
Yes — the division by the total makes the units cancel, so entering 0.7577 and 0.2423 gives the same average as 75.77 and 24.23.
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This calculator is for educational purposes. Double-check important results before acting on them.