Atomic Mass Calculator
Calculate an element's average atomic mass from isotope masses and natural abundances
Hit Calculate to see your result
Enter isotope abundances and calculate
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Small differences in isotope mass or abundance can change the final weighted average.
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Test how changing one isotope's abundance affects the average without changing your main result.
Calculate your baseline first
Multiply each isotope's mass by its decimal abundance, then add all contributions. A percentage such as 75.77% becomes 0.7577.
The periodic-table value is a weighted average of naturally occurring isotopes, not the mass number of a single atom.
Small rounding differences may be acceptable, but larger differences usually mean data is missing or entered incorrectly. This calculator warns you when the total differs materially from 100%.
Results are generated locally in your browser from the isotope masses and abundances you enter. They are educational estimates and may differ from published standard atomic weights because of rounding, sample variation or incomplete isotope data. Verify authoritative chemistry references before laboratory, academic or professional use.
