Calculate a weighted average from isotope masses and percentage abundances, with a strict 100% abundance check and worked calculation.
Method, examples and limits
For each isotope, enter its mass in u and its abundance in percent. The result is Σ(mass × abundance/100). Add up to ten rows. The tool requires the percentages to total 100% within a small numerical tolerance and does not automatically normalise them.
A hypothetical 20% population of mass 10 u and 80% population of mass 11 u yields 10.8 u. This average lies between the two isotope masses.
Use measured isotope masses when precision matters. Your result describes the entered sample composition and may differ from a standard atomic weight for natural terrestrial materials.
Worked learning guide
Average Atomic Mass from Isotope Abundance: A Worked Guide
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Frequently asked questions
Must isotope abundances add to 100%?
Yes. This tool rejects incomplete or excessive totals rather than silently normalising them.
Can an average lie outside the isotope mass range?
Not when all masses are weighted by non-negative fractions summing to one.