Average Atomic Mass Calculator
Calculate the average atomic mass of an element from its isotopes — enter each isotope’s mass and natural abundance to get the weighted average.
How to calculate average atomic mass
The average atomic mass of an element is the weighted average of its isotope masses, weighted by how abundant each isotope is in nature. Multiply each isotope’s mass by its abundance, add the results, and divide by the total abundance. With abundances entered as percentages, the calculator divides by 100 for you.
average atomic mass = Σ(isotope mass × abundance) ÷ Σ(abundance)
Chlorine, for example, is about 75.77% chlorine-35 (mass 34.969) and 24.23% chlorine-37 (mass 36.966): (34.969 × 75.77 + 36.966 × 24.23) ÷ 100 ≈ 35.45 amu — exactly the value on the periodic table. Enter as many isotopes as you like; the abundances don’t have to sum to exactly 100.
This is a weighted average — see the average calculator for the general weighted-average and mean tools.
Frequently asked questions
How do you calculate average atomic mass?
Multiply each isotope’s mass by its fractional abundance, then add the results. With abundances in percent, that is Σ(mass × abundance) ÷ Σ(abundance). For chlorine: (34.969 × 75.77 + 36.966 × 24.23) ÷ 100 ≈ 35.45 amu.
What is average atomic mass?
It is the weighted average mass of all naturally occurring isotopes of an element, weighted by how common each isotope is. It is the value shown on the periodic table, measured in atomic mass units (amu).
Why is chlorine’s atomic mass 35.45?
Chlorine is about 75.8% chlorine-35 and 24.2% chlorine-37. The weighted average of those isotope masses lands at roughly 35.45 amu — between the two but closer to the more abundant chlorine-35.