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What does this calculator estimate?
An average atomic mass calculator combines isotope masses and abundances into the weighted average. Enter each isotope as mass:abundance pairs to get the element's atomic weight.
- Average = Σ (mass × fractional abundance)
- Abundance can be % or fraction
- Matches periodic-table atomic weight
Why atomic weights aren't integers
Most elements exist as multiple isotopes — atoms with the same proton count but different neutron counts. The periodic-table atomic weight is the weighted average of those isotopes by natural abundance, which is why chlorine is 35.45, not 35.
Limitations to watch for
Abundances must sum to ~100% for a valid result; the calculator normalizes what you enter. Isotope masses are exact per nuclide, not rounded. Samples from different sources (meteorites, reactor products) can have slightly different ratios.
How to use it in practice
Look up the isotope masses and natural abundances for the element (standard reference tables), enter them as mass:abundance pairs, and compare the result to the periodic-table atomic weight. Use % for convenience — the tool converts.
['Enter each isotope as mass:abundance (e.g., 34.9689:75.78).', 'Separate pairs with commas or semicolons.', 'The tool computes the weighted average atomic mass.']
How this calculator works
Formula
Average atomic mass = Σ (isotope mass × fractional abundance). Enter isotopes as 'mass:abundance' pairs (abundance as % or fraction), separated by commas or semicolons. Example: '34.9689:75.78, 36.9659:24.22'.
Worked example
Chlorine has two stable isotopes: 34.9689 amu (75.78%) and 36.9659 amu (24.22%). Average = 34.9689 × 0.7578 + 36.9659 × 0.2422 ≈ 35.45 amu — the atomic weight on the periodic table.
Assumptions to verify
- The listed isotopes and abundances are correct.
- Abundances sum to 100% of the element's atoms.
- The calculation is the standard weighted average.
Frequently asked questions
What is average atomic mass?
The weighted average of an element's isotope masses by natural abundance. Chlorine's 35.45 amu comes from 75.78% ³⁵Cl and 24.22% ³⁷Cl.
How do I calculate it?
Multiply each isotope mass by its fractional abundance and add: 34.9689 × 0.7578 + 36.9659 × 0.2422 ≈ 35.45.
What is an isotope?
Atoms of the same element with different neutron counts — same protons, different mass. Carbon-12 and carbon-14 are isotopes of carbon.
Why isn't the atomic weight a whole number?
Because it's an average of multiple isotopes, not a single nuclide. Elements with one dominant isotope are close to integers (beryllium 9.012).
Can I use percentages instead of fractions?
Yes — the tool accepts abundances above 1 as percentages and converts them (75.78 → 0.7578).
What if abundances don't sum to 100%?
The result will be slightly off. Check that you've listed all significant isotopes.
Does this work for molecules?
No — this is for elements. Molecular weights come from the molar mass calculator.
Cite this tool
BoringToolsKit. “Average Atomic Mass Calculator.” boringtoolskit.com/average-atomic-mass-calculator/ (reviewed 2026-08-25). Free to reference in articles, syllabi, and answer posts with a link.
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