Average Atomic Mass Calculator

Your details

Standard mode: enter masses and abundances, get the average. Reverse mode: enter masses, all-but-one abundance, and the known average, and find the missing abundance.
Pre-fills the isotope fields with NIST-published masses and abundances. Switch to "Custom" to enter your own values.
How many isotopes to include in the calculation.
Percent is more common in textbooks; decimal fraction is used in some research papers. The calculator converts automatically.
Isotopic mass of the first isotope in atomic mass units.
amu
Natural abundance of isotope 1 (percent or decimal, depending on the setting above).
%
Isotopic mass of the second isotope in atomic mass units.
amu
Natural abundance of isotope 2. In reverse mode, leave this blank to solve for it.
%
When on, the entered abundances are scaled so they sum to exactly 100% before computing. Useful when textbook rounding leaves a small gap.
Average atomic massAbundances sum to 100%
35.45274amu
Total abundance entered100%
Isotope 1 contribution26.4959amu
Isotope 2 contribution8.95684amu
Isotope 1 contribution26.4959
Isotope 2 contribution8.95684

Average atomic mass = 35.45274 amu.

  • The abundance-weighted average atomic mass is 35.45274 amu.
  • Each isotope contributes in proportion to how common it is. The most abundant isotope pulls the average closest to its own mass.
  • Your abundances sum to 100%, giving an exact weighted average with no normalisation needed.

Next stepCompare this to the periodic-table value. Differences larger than 0.01 amu usually indicate a mass or abundance entry error.

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