Percent Composition Calculator

Calculate mass percent of each element in a chemical compound.

How It Works

Enter the molecular formula. The calculator determines the molar mass, then computes the percentage contribution of each element by dividing its total mass contribution by the molar mass and multiplying by 100.

Formula

% composition = (mass of element in formula / molar mass) × 100

Examples

H2O: H = 11.19%, O = 88.81% | NaCl: Na = 39.34%, Cl = 60.66%

Frequently Asked Questions

How do you calculate percent composition?

Divide the total mass of one element in the formula by the molar mass of the compound, then multiply by 100. Example: for H2O, oxygen is (15.999/18.015) × 100 = 88.81%.

Why does percent composition matter?

It lets chemists verify compound identity, determine empirical formulas, and calculate how much of each element is present in a given mass of compound.

Do all percentages add to 100?

Yes. The sum of all elemental mass percentages in a compound always equals 100% (allowing for rounding).