Average Atomic Mass Calculator
Enter each isotope’s mass and its natural abundance as a percent, and get the weighted average atomic mass with the working shown.
- Any number of isotopes
- Contribution per isotope
- Shows the working
- Free
Average atomic mass
Abundances are percentages, so they should add up to about 100.
| Isotope mass | Abundance | Share | Contribution |
|---|
Show the working
How to calculate average atomic mass
Turn each percent into a share
Chlorine-35 is 75.76% of natural chlorine, so its share is 0.7576. Chlorine-37 is 0.2424.
Multiply each mass by its share
34.96885 × 0.7576 = 26.49, and 36.96590 × 0.2424 = 8.96.
Add the pieces up
26.49 + 8.96 = 35.45 amu, the number printed on the periodic table for chlorine.
The answer always lands between the lightest and heaviest isotope, closer to whichever one is more common. That is a useful check on your own working.
Questions people ask
How do you calculate average atomic mass?
Multiply each isotope’s mass by its natural abundance written as a decimal share, then add the results. For chlorine: 34.96885 × 0.7576 + 36.96590 × 0.2424 = 35.45 amu.
Should I enter abundance as a percent or a decimal?
As a percent: 75.76, not 0.7576. The calculator does not guess between the two, because 0.778 could honestly mean 0.778%. If your figures are decimal shares, multiply each by 100 first.
What if my abundances do not add up to 100?
The calculator still answers, by dividing each abundance by the total you entered, and it tells you it did that. Be aware that the result is then the weighted average of the isotopes you listed, which is not the element’s true atomic weight unless you listed them all.
Why is the average atomic mass not a whole number?
Because it is an average across isotopes that occur in different proportions. Chlorine is a mix of chlorine-35 and chlorine-37, so its average of 35.45 sits between them and matches neither.
What is the difference between mass number and atomic mass?
The mass number is a whole-number count of protons plus neutrons in one particular isotope. The atomic mass is the measured mass of that isotope in amu, and the average atomic mass is the abundance-weighted average across all of them.
Where do I find isotope masses and abundances?
A chemistry textbook, a data booklet or an exam paper will give them. This calculator does not look them up, because the numbers in a question are sometimes deliberately different from the real ones.
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