Percent Composition Calculator
Calculate each element’s mass percentage from a formula, including grouped atoms and hydrates, with atomic weights and contribution calculations.
Find how much of a compound’s mass comes from each element using its chemical formula. The calculator expands repeated groups and hydrate components, then reports element contributions and mass percentages. This helps compare compositions without choosing a particular sample size.
How to use this tool
- Enter the formula, preserving capital and lowercase letters.
- Include all hydrate waters using a middle dot, for example CuSO4·5H2O.
- Check the expanded atom counts in the table.
- Read each element’s share of total mass and the formula’s molar mass.
Formula
mass percentᵢ = 100 × (atom countᵢ × atomic weightᵢ) / Σ(atom count × atomic weight)
The denominator includes every atom in the entered formula. All shares use the same CIAAW 2024 nominal atomic weights, so unrounded percentages sum to 100. Rounded display values may differ slightly.
Example
Water has two H atoms and one O atom. Their contributions are 2 × 1.008 = 2.016 and 15.999, totaling 18.015. Hydrogen therefore accounts for 100 × 2.016 / 18.015 = 11.1907% of the mass; oxygen accounts for 88.8093%.
Frequently asked questions
Is mass percentage the same as atom percentage?
No. Water has twice as many hydrogen atoms as oxygen atoms, but hydrogen contributes only about eleven percent of the mass because each hydrogen atom is much lighter.
Does an empirical formula give the same percentages?
Yes. Multiplying every atom count by the same number multiplies both numerator and denominator equally. CH2O and C6H12O6 therefore have identical elemental mass percentages.
How does hydration change the answer?
Water of crystallization adds hydrogen and oxygen mass to the denominator as well as those elements’ numerators. Omitting hydrate waters gives the composition of a different material.
What are the limits?
This is a formula calculation, not a purity assay or mixture analysis. Charges, isotope labels, decimal subscripts and unknown polymer counts are unsupported. Formula length is capped at 500 characters, nesting at twelve groups and expansion at one million atoms. Natural isotope variation and reference rounding affect the last digits; isotope-enriched materials require other data.
Sources
Last updated: 2026-10-30