Average Atomic Mass Calculator

The atomic mass on the periodic table is a weighted average over all naturally occurring isotopes. Enter each isotope's mass and its fractional abundance (as a decimal) to reproduce that table value — or solve backwards for a missing abundance.

Average Atomic Mass Calculator

$\bar{A} = f_1 m_1 + f_2 m_2$

Solve for

Fractional abundance of isotope 1 (f_1)

Unit

Isotopic mass 1 (m_1)

Unit

Isotopic mass 2 (m_2)

Unit

Result unit

How to interpret the result

Worked check: chlorine — ³⁵Cl at 75.78% (34.969 u) and ³⁷Cl at 24.22% (36.966 u): 0.7578×34.969 + 0.2422×36.966 = 26.500 + 8.953 = 35.45 u, the familiar table value.

Assumptions

Exactly two isotopes contribute (extend the sum for more)
Abundances are entered as decimal fractions summing to 1