Average Atomic Mass Calculator

This chemistry calculator computes the abundance-weighted average atomic mass from up to ten isotope masses and natural abundances.

Formula notes

average mass = sum(mass_i x abundance_i / 100), with abundances totaling 100%.

Worked examples

1. Example: Cl-35 at 34.96885 u with 75.77% and Cl-37 at 36.96590 u with 24.23% gives about 35.45 u.
2. Neon with 90.48% Ne-20 at 19.99244 u and 9.25% Ne-22 at 21.99138 u gives an average near 20.18 u.

How to use it

1. Enter up to ten isotope masses separated by commas.
2. Enter one natural abundance for each mass.
3. Read isotope count, total abundance, and average atomic mass.

Frequently asked questions

Limits and interpretation

• The result uses only the masses and abundances you enter; natural isotope ratios can vary by sample source.
• Values are rounded to the precision of the inputs and are not a substitute for calibrated mass-spectrometry measurements.

References

• OpenStax Chemistry 2e