Average Atomic Mass Calculator
The weighted average of isotope masses — the number printed on the periodic table.
Average atomic mass
35.4527 u
That matches Chlorine (Cl), listed as 35.45 on the periodic table.
Working
- 34.96885 u × 75.77% = 26.4959
- 36.9659 u × 24.23% = 8.95684
- Average = Σ(mass × abundance) ÷ 100.00 = 35.4527 u
Weighted, not plain, averages
The most common error here is averaging the isotope masses without weighting them. For chlorine that gives 35.97, which is nowhere near the true 35.45 — the light isotope is three times as common, and the average has to lean towards it.
Fractions or percentages
Abundances can be entered as percentages, which is how problems usually state them. If yours are given as decimal fractions, multiply by 100 first, or simply enter them consistently: what matters is that the weights are in the same units as each other.
Related: molar mass calculator, chemical equation balancer, periodic table.
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Frequently Asked Questions
What is average atomic mass?
It is the weighted mean of the masses of an element’s naturally occurring isotopes, weighted by how common each one is. It is the number printed on the periodic table.
How do you calculate it?
Multiply each isotope’s mass by its fractional abundance and add the results. For chlorine: 34.969 × 0.7577 plus 36.966 × 0.2423 gives 35.45.
Why is chlorine 35.45 and not a whole number?
Because no single chlorine atom weighs 35.45. Natural chlorine is about 76% chlorine-35 and 24% chlorine-37, and 35.45 is what a large sample averages to. Averages of whole numbers need not be whole.
Should abundances add up to 100%?
Yes, if you have listed every isotope. This calculator divides by whatever total you enter, so the weighting stays correct either way, but a total well short of 100% usually means an isotope is missing.
Why do some elements have no average atomic mass?
Elements with no stable isotope — technetium, promethium and everything past bismuth — have no fixed natural abundance to average over. The periodic table shows the mass number of the longest-lived isotope in brackets instead.