Chemistry · Composition

Percent composition calculator.

Mass percent of each element in a compound, and the grams of each element in a sample you weigh out.

Percent by massGrams per sampleCIAAW atomic weights

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How to calculate percent composition by mass

Percent composition is the share of a compound's mass that comes from each element: mass percent = (element mass in one mole of compound / molar mass of the compound) x 100. The percentages add to 100%.

Worked example. NaCl: Na 22.99 + Cl 35.45 = 58.44 g/mol. Sodium is 22.99 / 58.44 = 39.34% and chlorine is 60.66%. In a 25 g sample that is 9.835 g of sodium and 15.17 g of chlorine. The formula C6H12O6 (glucose) has molar mass 180.16 g/mol, of which carbon is 40%.

This is the theoretical composition of the pure formula. A real sample with moisture, a different hydrate or impurities will not match it.

Frequently asked questions

What is percent composition?

It is the percentage of the total mass of a compound that each element contributes, worked out from the formula and the atomic weights.

How do I find the mass of an element in a sample?

Multiply the sample mass by the element's mass percent and divide by 100. Enter the sample mass above to see it for every element.

Is percent composition the same as percent by atoms?

No. It is by mass. Water is 2 hydrogen atoms to 1 oxygen atom, but oxygen is about 89% of the mass.

Does it work for hydrates?

Yes. Enter the formula with a dot, such as CuSO4.5H2O, and the water is counted in the total.

Sources

Checked8 October 2026

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Lab safety and scope

This is arithmetic, not professional advice. Check every result against your own procedure, the safety data sheet (SDS) for each chemical, and your lab supervisor or instructor before you prepare, mix or dispose of anything. Verify values on the reagent bottle, since purity and hydration state change the real molar mass.