Atomic Mass Calculator

Or click on an element from the periodic table below

Periodic Table (Common Elements)

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How it works

Atomic mass is the weighted average mass of atoms of an element, measured in atomic mass units (amu). It takes into account the relative abundances of different isotopes of the element.

The formula for atomic mass is:

Atomic Mass = Σ (Isotope Mass × Relative Abundance)

Where:

  • Isotope Mass = Mass of each isotope
  • Relative Abundance = Fractional abundance of each isotope (as a decimal)

For example, carbon has two main isotopes:

  • Carbon-12 (98.93% abundance, mass = 12.000 amu)
  • Carbon-13 (1.07% abundance, mass = 13.003 amu)

Atomic mass = (12.000 × 0.9893) + (13.003 × 0.0107) = 12.011 amu

The values shown in this calculator are based on IUPAC standard atomic weights.