Average Atomic Mass Calculator
The average atomic mass of an element from the masses and natural abundances of up to four isotopes — the number printed on the periodic table.
Multiply each isotope’s mass by its fractional abundance and add them up.
How the average atomic mass calculator works
Multiply each isotope’s mass by its fractional abundance and add them up. Chlorine is the textbook case: 75.76% of ³⁵Cl at 34.969 u and 24.24% of ³⁷Cl at 36.966 u average to 35.45 u, which is why chlorine’s atomic mass is not a whole number. Abundances should add to 100%; the calculator warns when they are more than 1% off.
Formula: average mass = Σ (isotope mass × abundance ÷ 100)
Worked examples
| Inputs | Average atomic mass | Note |
|---|---|---|
| Chlorine: ³⁵Cl 75.76%, ³⁷Cl 24.24% | 35.4529 u | 35.4529 u |
| Copper: ⁶³Cu 69.15%, ⁶⁵Cu 30.85% | 63.546 u | 63.546 u |
| Magnesium, three isotopes | 24.3051 u | 24.305 u |
FAQFrequently asked questions
Why is the periodic-table mass not a whole number?
Because it averages several isotopes weighted by abundance, and because nuclear binding energy puts even a single isotope’s mass slightly off an integer. Carbon-12 is defined as exactly 12 u.
Where do I find isotope masses and abundances?
IUPAC’s Commission on Isotopic Abundances and Atomic Weights publishes them, and most chemistry textbooks tabulate the common elements. Masses are in unified atomic mass units (u).
What if the abundances do not add to 100%?
Rounding can leave them a fraction off, which the calculator accepts; more than 1% out means an isotope is missing or a figure is wrong, so it stops rather than give a misleading average.
Does this give the periodic-table value exactly?
Very close. Small differences arise because IUPAC quotes a range for elements whose isotope ratios vary between natural samples, such as lithium, boron and carbon.
Where these figures come from
- IUPAC — Standard atomic weights (2021 conventional values) — the molar-mass table
- NIST — CODATA 2018 fundamental physical constants — Avogadro constant, gas constant, speed of light
- NIST Chemistry WebBook — thermochemical data
- CSIRO — Australia's national science agency
Last checked: September 2026. Atomic masses are the IUPAC conventional values; constants are CODATA 2018; equations are the standard textbook forms.