Average Atomic Mass Calculator
Calculate an abundance-weighted atomic mass from any list of isotope masses and percentages.
01 Set up your calculation
- List each isotope mass in u, separated by commas.
- List its percentage abundance in the same order; the percentages must total 100.
- Use measured masses rather than integer mass numbers when precision matters.
02 The relationship
For the assumptions behind these results, read the explanation below. You can also explore Atom and Atomic Mass.
Follow the numbers
Using 34.96885 u at 75.76% and 36.96590 u at 24.24% gives 35.45294 u. The more abundant lighter isotope pulls the mean toward its mass.
03 The percentages are weights
A mixture containing mostly one isotope has a mean close to that isotope’s mass. An unweighted mean gives the right answer only when the listed isotopes are equally abundant. Keep the mass list and abundance list aligned.
Zero abundance is allowed for an isotope you want to keep in the list. If one isotope has 100% abundance, the mean is simply that isotope’s mass.
04 Represent the actual sample
A standard atomic weight summarizes a particular convention or range of natural compositions. Enriched, depleted, or geographically unusual material can have a different mean. Enter the distribution relevant to the sample you are studying.
The calculator does not silently normalize incomplete percentages. If rounded measurements total 99.9%, first decide how to resolve that discrepancy. Treating a partial list as a complete sample would hide missing material.
Questions from the notebook
Can I enter fractions instead of percentages?
Multiply each fraction by 100 first. Values such as 0.75 and 0.25 describe percentages totaling 1, so this tool rejects them.
Can the mean lie outside the isotope masses?
With valid nonnegative percentages it cannot. The mean lies between the smallest and largest mass with nonzero abundance.
References & reading
- OpenStax, Rice University — Chemistry 2e: Atomic Structure and Symbolism ↗
Defines atomic number, mass number, ions and isotope-weighted atomic masses.
- NIST — Atomic Weights and Isotopic Compositions ↗
Lists isotope masses and standard atomic weights. Formula calculations use rounded conventional values, not isotope-specific exact masses.
Our explanations are written for ChemistryCalc. Read how we build and check our tools. These calculations are educational estimates where the underlying model requires assumptions.