Skip to main content
Periodixy

Percent Composition Calculator

Enter a formula and see what percent of the compound's mass each element contributes — displayed as a table and proportional bars, computed from current atomic masses.

What percent composition means

Formula

% element = (mass of element in one mole ÷ molar mass of compound) × 100

Percent composition answers “how much of this compound's mass comes from each element?” It's how fertiliser bags advertise nitrogen content, how ores are graded, and the standard first step when chemists work backwards from an unknown compound to its empirical formula.

Worked example: water

What percent of water's mass is oxygen?

  1. Molar mass of H₂O ≈ 18.02 g/mol.
  2. Oxygen contributes 15.999 g/mol.
  3. % O = 15.999 ÷ 18.02 × 100

Answer: ≈ 88.8% oxygen (and 11.2% hydrogen) — most of water's mass is oxygen.

Things worth noticing

  • Percentages always total 100% (within rounding) — a quick self-check for manual work.
  • Atom count ≠ mass percent: water is ⅔ hydrogen atoms but only 11% hydrogen by mass, because oxygen atoms are 16× heavier.
  • Hydrates split naturally: in CuSO₄·5H₂O, the water of crystallisation is ~36% of the mass — which is why heating blue copper sulfate loses over a third of its weight.

Frequently asked questions

What is the percent composition of CO2?

About 27.3% carbon and 72.7% oxygen by mass — carbon contributes 12.01 of the 44.01 g/mol total.

Why do my percentages add to 99.9% or 100.1%?

Rounding at intermediate steps. Carry an extra decimal through the calculation and round only at the end.

How is percent composition used to find formulas?

Treat the percentages as grams in a 100 g sample, convert to moles, and reduce to the simplest ratio — exactly what the empirical formula calculator does.

Related chemistry tools

Molar Mass CalculatorCalculate the molar mass of any chemical formula with a full element-by-element breakdown and percent composition.Empirical Formula CalculatorTurn masses or percent composition into the simplest whole-number formula, step by step.Interactive Periodic TableExplore all 118 elements with hover details, filters, trends, temperature states, a formula builder and more.

Learn the concepts

Molar Mass: Definition and How to Calculate ItWhat is molar mass, and how do you find it? Add up the atomic masses in a formula to get grams per mole. Worked examples for H2O, glucose, Ca(OH)2 and NaCl.How to Read Chemical FormulasDecode any chemical formula: element symbols, subscripts, parentheses, coefficients and hydrate dots — with H2O, Ca(OH)2 and CuSO4·5H2O examples.