Chemistry
Average Atomic Mass Calculator
Combine isotope masses and their natural abundances into the average atomic mass of an element.
Your inputs
Change any value to update the result instantly.
Average atomic mass
35.4529 u
Total abundance entered
100 %
Isotope 1 contribution
26.4924 u
Isotope 2 contribution
8.9605 u
Isotope 3 contribution
0 u
What this calculator does
Computes the abundance-weighted average atomic mass of an element from up to three isotopes.
What your result means
The weighted mean isotopic mass in u — the value quoted on the periodic table for that element.
Formula and working
AM = Σ(fᵢ × mᵢ)
- 1
Collect the inputs
Isotope 1 mass = 34.9689 u; Isotope 1 abundance = 75.76 %; Isotope 2 mass = 36.9659 u; Isotope 2 abundance = 24.24 %; Isotope 3 mass (optional) = 0 u; Isotope 3 abundance = 0 %
- 2
Apply the formula
AM = Σ(fᵢ × mᵢ)
- 3
Result
Average atomic mass = 35.4529 u
Frequently asked questions
Symbols and assumptions
- fᵢ
- Fractional abundance of isotope i
- mᵢ
- Mass of isotope i in u
- AM
- Σ(fᵢ × mᵢ)
- • Abundances are natural-abundance percentages and are normalised to 100% before weighting.
Sources and review
Reviewed 2026-09-22 · Next review 2027-09-22
- • Samples with non-natural isotope ratios (enriched or depleted material) give different averages.
Important limitation
This is an educational estimate based only on the values and formula shown. Verify current rates, rules, units, and professional standards before relying on the result. It is not personalized financial or investment advice.
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