Atomic Mass Calculator With Abundance

Updated 4 November 2026 · Standard formula, shown below

Average atomic mass 35.4529 u

Abundances entered add up to: 100.00 %; Result
Mass of isotope 1: 34.96885 u; You entered
Abundance of isotope 1: 75.76 %; You entered

This calculator works out the average atomic mass of an element from its isotopes. Enter the mass of each isotope in atomic mass units and its natural abundance in percent, and you get the abundance-weighted average mass.

Formula

Average atomic mass = (m₁ × A₁ + m₂ × A₂) ÷ (A₁ + A₂). With abundances in percent that add up to 100, this is the same as m₁ × A₁/100 + m₂ × A₂/100.

m₁, m₂: isotope masses in unified atomic mass units (u, also written amu or Da)
A₁, A₂: natural abundances in percent

Worked Example

Chlorine: chlorine-35 has mass 34.96885 u and abundance 75.76%; chlorine-37 has mass 36.96590 u and abundance 24.24%.

34.96885 × 75.76 = 2649.2401
36.96590 × 24.24 = 896.0534
Sum = 3545.2935; the abundances add up to 75.76 + 24.24 = 100.00%
3545.2935 ÷ 100
Average atomic mass = 35.4529 u (the periodic table lists about 35.45)

Limits and common mistakes

Dividing by the abundance total means the result is still right if your percentages add to 99.9 or 100.1 from rounding; check the total line anyway.
Use isotope masses, not mass numbers (35 and 37), for an accurate result.
For an element with three isotopes add the third m × A term to the top and its A to the bottom.

Formula used

in m1 | Mass of isotope 1 | u | 34.96885 | 0
in a1 | Abundance of isotope 1 | % | 75.76 | 0
in m2 | Mass of isotope 2 | u | 36.96590 | 0
in a2 | Abundance of isotope 2 | % | 24.24 | 0
out am | Average atomic mass | u | (m1 * a1 + m2 * a2) / (a1 + a2) | 4
out tot | Abundances entered add up to | % | a1 + a2 | 2

Each line is one input (in, sel, list) or one result (out: label, unit, formula, decimals). How the calculators work

Method: the standard formula for this calculation, rounded as each result states. See how the calculators work or report an error.