Percent Composition Calculator
Find the mass percent of every element in a compound from its chemical formula.
How it works
mass % of element = (element mass in formula ÷ total molar mass) × 100
Percent composition tells you what fraction of a compound's mass comes from each element. For every element the calculator multiplies its atomic mass by how many atoms appear in the formula, divides by the compound's total molar mass, and multiplies by 100. The percentages always add up to 100.
Worked example
For water, H₂O (18.015 g/mol): oxygen is 15.999 ÷ 18.015 × 100 ≈ 88.81%, and the two hydrogens are 2.016 ÷ 18.015 × 100 ≈ 11.19%.
Frequently asked questions
Why do the percentages add up to 100?
Because every atom's mass is counted once and divided by the same total. The parts always sum to the whole, so rounding aside the columns total 100%.
How is this different from empirical formula?
Percent composition goes from a known formula to mass fractions. The empirical formula calculator does the reverse — it turns measured mass percentages back into the simplest whole-number formula.
Can I use it for hydrates?
Yes. Write the hydrate with its water of crystallisation, for example CuSO4(H2O)5, and the parser includes the water in the composition.
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