🧪 Average Atomic Mass Calculator
Determine the weighted atomic mass from isotopic masses and percent abundances.
Enter up to three isotopes. Use precise isotopic masses (in atomic mass units) and corresponding percent abundances. Leave unused rows blank.
How to Use This Calculator
Collect isotope data
Use a data table or your experimental measurements to obtain isotopic masses (in atomic mass units) and percent abundances.
Enter each isotope
Fill a row with the isotopic mass and its relative abundance. Leave unused rows blank if fewer than three isotopes are present.
Run the calculation
The calculator multiplies each mass by its fractional abundance and sums the contributions, automatically normalising abundances when necessary.
Record the result
Use the weighted average mass for stoichiometry, isotopic labelling analysis, or to verify periodic table values.
Formula
\u003C m \u003E = \u2211i (mi \u00d7 fi)
The weighted average atomic mass is found by multiplying each isotopic mass (mi) by its fractional abundance (fi = %/100) and summing the results. When abundances do not sum to 100%, they are normalised so that the fractions add up to 1.
Example: Chlorine
Isotopes: 35Cl (34.96885 u, 75.78%), 37Cl (36.96590 u, 24.22%)
<m> = 34.96885 × 0.7578 + 36.96590 × 0.2422 = 35.453 u
Full Description
Elements found in nature often exist as mixtures of isotopes, each with slightly different masses. The average atomic mass listed on the periodic table reflects these mixtures. This calculator simplifies the weighted-average computation by accepting isotopic masses and abundances, automatically normalising the inputs, and returning a precise average suitable for stoichiometry, spectrometry interpretation, and educational exercises.
Because isotopic abundances can vary by source or sample (e.g., terrestrial vs. extraterrestrial materials), the tool highlights when abundances do not sum to 100% and adjusts them proportionally. This feature is particularly helpful when using raw mass spectrometry data that may still require scaling. The responsive layout and intuitive validation make it quick to run multiple scenarios while discussing isotopic effects on atomic masses, nuclear stability, or environmental isotope tracing.
Common applications
- Chemistry education: Demonstrate the connection between isotopes and periodic table atomic weights.
- Mass spectrometry: Convert intensity ratios into average atomic masses for sample analysis.
- Geochemistry and climatology: Compare isotopic signatures in environmental samples.
- Pharmaceutical research: Evaluate isotopic labelling in tracer studies or metabolomics.
- Quality control: Document isotopic composition for materials with regulated specifications.
Why scientists trust this tool
- ✅ Flexible input: Accepts any number of isotopes up to three and ignores blank rows.
- ✅ Automatic normalisation: Adjusts abundances so the math always works even with raw instrument data.
- ✅ Precision output: Reports five decimal places to match modern analytical accuracy.
- ✅ Concept reinforcement: Encourages understanding of weighted averages rather than memorisation.
- ✅ Cross-platform: Works on desktop and mobile devices during lectures or lab sessions.
Frequently Asked Questions
Do abundances need to add up to 100%?
No. The calculator automatically normalises abundances so that their sum equals 100%. A note appears whenever normalisation occurs, making it clear that scaling was applied.
Can I enter fractional abundances instead of percentages?
Yes. You can enter fractions (e.g., 0.7578) and the calculator will treat them as percentages once the normalisation step runs. Just be consistent across all entries.
What if my sample has more than three isotopes?
You can combine minor isotopes into a single entry using a weighted average, or rerun the calculation with different groupings. Future updates may allow additional rows based on feedback.
Where do I find accurate isotopic masses?
Refer to NIST data tables, IUPAC evaluations, or high-resolution mass spectrometry outputs. Precision matters because even small mass differences influence the final average.
Can I use this for radioactive isotopes?
Absolutely. The tool treats any isotope mass and abundance numerically, whether the isotope is stable or radioactive. Ensure that the abundance values reflect your specific sample.