Average Atomic Mass Calculator

Find average atomic mass from two isotopes.

Isotope 1 mass (amu)

Isotope 1 abundance (%)

Isotope 2 mass (amu)

Isotope 2 abundance (%)

Average atomic mass (amu) 35.453

Formula: avg = Σ(mass × abundance) ÷ 100

Step-by-step with your numbers:

1. Values used:

2. Isotope 1 mass = 34.969 amu

3. Isotope 1 abundance = 75.77 %

4. Isotope 2 mass = 36.966 amu

5. Isotope 2 abundance = 24.23 %

7. Average atomic mass = 35.453amu

Did we solve your problem today?

An element's atomic mass is the abundance-weighted average of its isotopes.

The math behind it

average = Σ(isotope mass × % abundance) ÷ 100.

Chlorine: 34.969×75.77% + 36.966×24.23% → 35.45 amu.

Why not a whole number?

It averages isotopes of different masses.