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Percent Composition Calculator

Find the mass percentage of a component.

How it is calculated

The tool divides the mass of one component by the total mass and multiplies by 100: percent = part / total × 100. Both masses must be in the same unit (grams, kilograms or atomic mass units) and the unit cancels out, so the answer is a pure percentage shown to two decimals. For a compound, the part is the combined mass of all atoms of that element in one formula unit, for example 2 × 1.008 for the hydrogen in H₂O, and the total is the molar mass of the whole formula. The total must be above zero. The tool does not check that the part is smaller than the total, so a part larger than the total returns a figure above 100%, and a negative part returns a negative percentage.

Worked example

What share of water's mass is oxygen? One mole of H₂O has a molar mass of 18.015 g, and the oxygen in it contributes 15.999 g. Divide: 15.999 / 18.015 = 0.8881. Multiply by 100 to get 88.81%. The hydrogen is the rest: 2.016 / 18.015 gives 11.19%, and the two percentages add up to 100%. So about 89% of the mass of any water sample is oxygen, even though there are twice as many hydrogen atoms.

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Common mistakes

  • Using the atom count instead of the mass. In H₂O there are two hydrogens and one oxygen, but percent composition uses their masses, and hydrogen is only 11.19% by mass.
  • Forgetting to multiply the atomic mass by the subscript. For hydrogen in water the part is 2 × 1.008 = 2.016, not 1.008; using 1.008 gives 5.60% rather than 11.19%.
  • Entering the part and total in different units, such as milligrams against grams. The tool cannot tell, so a part of 500 mg against a total of 2 g must be entered as 0.5 and 2.
  • Swapping part and total. A result above 100% is a signal that the two numbers are in the wrong fields, since the tool does not warn you.
  • Using a rounded or wrong molar mass for the total. If the total is not the sum of the parts, the percentages of the components will not add up to 100%.

About this calculator

Percent composition (mass percent) is simply (part / total) × 100 — the mass of one component divided by the total mass of the mixture or compound. In chemistry, it's most often used to describe the percentage by mass of each element within a compound, calculated from the elements' molar masses relative to the compound's total molar mass, though the same math applies just as well to mixture purity or ingredient proportions.

Chemistry students use percent composition to verify empirical formulas from experimental mass data, analytical chemists use it to report purity of a sample or product, food scientists use it to calculate nutrition-label percentages, and pharmacists use it when formulating mixtures to hit a target concentration by mass.

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Comments (1)

  • Editorial team

    Percent composition also works backwards, which is how lab results become formulas. Treat each percentage as grams in a 100 g sample and divide by the element's atomic mass. For 88.81 and 11.19 percent oxygen and hydrogen, that is 88.81 ÷ 15.999 = 5.551 and 11.19 ÷ 1.008 = 11.10 moles, a 1 to 2 ratio, so H₂O. What kind of sample are you working from, a textbook compound or your own lab data?

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