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Isotope Abundance Calculator
Calculate weighted average atomic mass and isotopic percentages for a set of natural isotope abundances.
This works right here on your device — nothing gets sent out.
Leave exactly one abundance blank to solve for it (max 10 isotopes).
How it works
- Enter each isotope's mass (in u) and its natural abundance (in %), or load an example such as chlorine or boron.
- Add up to 10 isotopes. Leave one abundance blank to solve for it automatically.
- Use Normalize to 100% if your abundances are close but don't quite add up.
- Read the weighted average atomic mass and each isotope's contribution, then copy the result.
The formula
Average atomic mass is the abundance-weighted mean of the isotope masses. Abundances are converted from percent to fractions.
Average mass = Σ (isotope mass × abundance ÷ 100) Missing abundance = 100 − Σ (known abundances)
Example: chlorine-35 (34.96885 u, 75.76%) and chlorine-37 (36.96590 u, 24.24%) give 26.4917 + 8.9612 = 35.4529 u, matching the standard atomic weight of about 35.45.
FAQ
How do you calculate average atomic mass from isotope abundance?
Multiply each isotope's mass by its fractional abundance (percent divided by 100), then add the results together.
How do I find a missing isotope abundance?
Leave that abundance blank. Because abundances must total 100%, the calculator subtracts the sum of the others from 100 and uses the result.
Why must the abundances add up to 100%?
Each abundance is a share of all atoms of the element, so the shares must total 100%. If yours are slightly off from rounding, use Normalize to 100%.
Why is average atomic mass not a whole number?
It is a weighted mix of isotopes with different masses, and each isotope's exact mass is itself slightly different from its whole-number mass number.
Where do I find isotope masses and abundances?
Use a reference source such as the NIST or IUPAC tables. The examples built into this tool use NIST-style values and reproduce the standard atomic weights.
Is my data sent anywhere?
No. Everything is calculated in your browser and nothing is uploaded.
How we compare
| Feature | Online Tool Store | Pearson | Omni Calculator | Calculator Academy |
|---|---|---|---|---|
| Solve one missing abundance | ✓ | ✓ | Not offered | ✓ |
| Normalize abundances to 100% | ✓ | ✓ | Not offered | ✓ |
| One-click element examples | ✓ | ✓ | Not offered | Not offered |
| Up to 10 isotopes with per-isotope contributions and copy | ✓ | Up to 6 isotopes | Up to 10 isotopes, no contribution breakdown | Up to 5 isotopes |
Pearson also identifies an element from its average mass and solves two unknown abundances, which this tool doesn't. For computing an average mass, filling a missing abundance, and checking the working, Isotope Abundance Calculator covers the everyday cases without a sign-up.