Average Atomic Mass Calculator
Use this average atomic mass calculator to combine isotope masses and abundances into a weighted average atomic mass for an element sample.
--
Run the calculator.
What This Average Atomic Mass Calculator Helps You Do
Average atomic mass is a weighted-average problem at heart, so this page is designed to make the isotope-abundance relationship explicit instead of hiding it behind a single periodic-table value.
That is useful for chemistry homework, isotope data checks, and any situation where you need to understand why an element's listed atomic mass sits between the masses of its isotopes.
How to Calculate Average Atomic Mass Calculator
- Enter isotope masses: Use the mass of each isotope in atomic mass units.
- Enter abundances: Use percentages or any proportional abundance values.
- Calculate the weighted average: The page multiplies each mass by its abundance share and divides by the abundance total.
- Check the abundance total: If the abundances do not add to 100%, the page still normalizes them and tells you the total used.
Average Atomic Mass Calculator Formula
| Variable | Meaning | Unit |
|---|---|---|
| m | Isotope mass | u |
| a | Isotope abundance | % or fraction |
Use the worked examples below to check how the formula behaves with real values. If the result looks unexpected, verify the unit assumptions and the meaning of each variable before interpreting the answer.
Worked Examples
- Isotope 1: 34.96885 u at 75.77%
- Isotope 2: 36.96590 u at 24.23%
Result: Average atomic mass is 35.4527 u.
Chlorine's average is closer to the lighter isotope because that isotope is more abundant.
- Isotope 1: 10.01294 u at 19.9%
- Isotope 2: 11.00931 u at 80.1%
Result: Average atomic mass is 10.8110 u.
The heavier isotope dominates the weighted average because it is far more abundant.
- Isotope 1: 62.92960 u at 69.17%
- Isotope 2: 64.92779 u at 30.83%
Result: Average atomic mass is 63.5456 u.
A weighted average lands between the isotope masses and leans toward the more abundant isotope.
- Isotope 1: 78.91834 u at 50.69%
- Isotope 2: 80.91629 u at 49.31%
Result: Average atomic mass is 79.9035 u.
Near-equal abundances produce an average near the midpoint of the isotope masses.
How to Interpret Your Results
| Range | Meaning | Action |
|---|---|---|
| Abundance total = 100% | The isotope abundances already form a complete distribution. | The weighted average can be read directly. |
| Abundance total not equal to 100% | The entries are still usable but need normalization. | Check whether the abundances were rounded or incomplete. |
| Average near one isotope mass | That isotope dominates the sample. | Expect its abundance to be the largest share. |
Frequently Asked Questions
References
Last reviewed: March 2026