How this calculator works
Pair every isotope mass with its percentage abundance. Multiply each mass by its fractional abundance and sum the contributions. Percentages must total 100% within one millionth of a percentage point; the calculator does not silently rescale incomplete samples. Isotope mass data describe the sample, so an enriched material can differ substantially from a standard atomic weight.
Formula
Average mass = sum of isotope mass x abundance / 100. Average mass in kg = average mass in u x atomic mass constant.
Worked example
A mixture of 20% mass-10 and 80% mass-11 atoms averages 10.8 u. A mixture with masses 24, 25 and 26 u at 80%, 10% and 10% averages 24.3 u.
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Frequently asked questions
Can an isotope have zero abundance?
Yes. It contributes zero, but all listed percentages still need to total 100%.
Should I use mass numbers?
Use measured isotope masses for precision. Mass numbers provide a rough exercise estimate.
Why two lists?
The first mass pairs with the first abundance, and so on. Both lists must have the same length.