How this calculator works
Expand the formula into element counts, calculate each mass contribution and divide by the whole formula mass. The table identifies elements in the same order as the chart. Hydrates include their water of crystallization. IUPAC 2021 conventional weights are used, so enriched isotopes need their own masses. The result describes mass fractions, not percentages of atoms.
Formula
Element mass percentage = 100 x atom count x atomic weight / relative formula mass.
Worked example
CO2 has molar mass 44.009 g/mol, of which carbon contributes 12.011 g/mol, or about 27.2912%. C6H12O6 totals 180.156 g/mol across 24 atoms.
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Frequently asked questions
Must the percentages add to 100%?
The unrounded values do. Display rounding can make their visible sum differ slightly.
Does the chart show atom fractions?
No. Oxygen and hydrogen atoms have different masses, so mass fractions differ from atom fractions.
Can I enter a hydrate?
Yes. Parentheses, square brackets and hydrate dots follow the same rules as the molar mass tool.