Average Atomic Mass Calculator
Find an element's average atomic mass from its isotopes. Enter each isotope's mass (in u) and its percent abundance for up to three isotopes; the abundances must total 100%. Leave unused rows at zero.
Example: with Isotope 1 mass (u) 34.96885 · Isotope 1 abundance (%) 75.76 · Isotope 2 mass (u) 36.9659 · Isotope 2 abundance (%) 24.24 · Isotope 3 mass (u) 0 → Average atomic mass: 35.4529 u.
- Abundance check100% total abundance
- Most abundant isotopeIsotope 1 (mass 34.96885 u) is most abundant at 75.76%
Computed by the calculator below using its default values. Change any input to see your own numbers.
Average atomic mass = Σ (isotope mass × fractional abundance). It is a weighted average, so the most common isotope pulls the result toward its own mass.
A weighted average, not a plain one
The atomic mass on the periodic table is not any single atom's mass; it is the average over all the naturally occurring isotopes, weighted by how common each one is. Chlorine, for instance, is a blend of chlorine-35 (about 75.8%) and chlorine-37 (about 24.2%). Averaging with those weights gives 35.45 u — closer to 35 because the lighter isotope is far more abundant.
To compute it, turn each percent abundance into a fraction, multiply by that isotope's mass, and add the products. The abundances have to sum to 100% for the average to be meaningful, which is why this tool flags any set that does not. A simple mean of the isotope masses would be wrong unless the isotopes happened to be equally abundant.
How it’s calculated
Average atomic mass = Σ (mᵢ × aᵢ/100), summed over each isotope with mass mᵢ (u) and percent abundance aᵢ. The abundances are required to total 100% within 1 percentage point; outside that range the tool reports an error instead of a mass.
Only naturally occurring isotopes with nonzero abundance are included. Isotope masses are taken as entered, in unified atomic mass units (u).
Average atomic mass of sample elements
| Element | Isotopes (mass, abundance) | Average mass |
|---|---|---|
| Chlorine | 35Cl 75.76%, 37Cl 24.24% | 35.45 u |
| Boron | 10B 19.9%, 11B 80.1% | 10.81 u |
| Copper | 63Cu 69.15%, 65Cu 30.85% | 63.55 u |
| Bromine | 79Br 50.69%, 81Br 49.31% | 79.90 u |
Computed as Σ(mass × abundance) from IUPAC isotope data; rounded.
Common mistakes
- Taking a plain average of the isotope masses instead of weighting by abundance.
- Leaving abundances as percents in the sum without dividing by 100.
- Entering abundances that do not total 100%, which skews the weighted mass.
- Swapping mass number (an integer like 35) for the precise isotopic mass (34.96885 u).
Frequently asked questions
How do you calculate average atomic mass?
Multiply each isotope's mass by its fractional abundance (percent ÷ 100), then add the results. It is a weighted average, so more abundant isotopes count more.
Why must the abundances add to 100%?
The fractional abundances are weights that have to cover every atom of the element. If they do not sum to 100%, the weighting is incomplete and the average is wrong.
Why is chlorine's mass 35.45 and not 36?
Chlorine-35 makes up about 76% of atoms and chlorine-37 only 24%, so the weighted average sits much closer to 35 than a simple midpoint would.
What units is atomic mass in?
Unified atomic mass units, u (also called daltons). One u is one-twelfth the mass of a carbon-12 atom.