Percent Composition Calculator

Percent composition by mass tells you what fraction of a compound's total molar mass is contributed by each element. It is one of the most fundamental analyses in chemistry because it links a compound's formula to measurable physical quantities, specifically the masses of the individual elements. The formula for each element is straightforward: multiply the number of atoms of that element in one formula unit by the element's molar mass to get the element's contribution to the molar mass, then divide by the compound's total molar mass and multiply by 100. The percentages of all elements must sum to exactly 100 percent. This calculator accepts up to six elements. For each element you enter a symbol (for labelling), the number of atoms in the formula and the molar mass of that element in grams per mole. Common molar masses: H = 1.008, C = 12.011, N = 14.007, O = 15.999, Na = 22.990, Cl = 35.45, Ca = 40.078. The calculator returns the molar mass contribution of each element, the compound's total molar mass and the percentage by mass for each element. The worked example is pre-filled for water (H2O) showing hydrogen at 11.19% and oxygen at 88.81%. Percent composition data are used to verify molecular formulas from synthesis, to identify unknowns by elemental analysis, and as the starting point for deriving empirical formulas.

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Enter up to 6 elements (symbol, number of atoms, molar mass g/mol):

18.015 g/mol
molar mass of compound
ElementAtomsMM (g/mol)Contribution (g/mol)% by mass

Molar masses: H = 1.008, C = 12.011, N = 14.007, O = 15.999, Na = 22.990, Cl = 35.45, Ca = 40.078, Fe = 55.845, S = 32.06, P = 30.974.

How it works

For each element i, the mass contribution is: contribution_i = atoms_i x molar_mass_i. The total molar mass M = sum of all contributions. The percentage by mass of element i is: % = (contribution_i / M) x 100. The percentages sum to 100 percent (within rounding). To find the empirical formula from percent composition, divide each percentage by the element's molar mass to get a mole ratio, then simplify to the smallest whole-number ratio.

Worked example

Water (H2O): Hydrogen (H) has 2 atoms at 1.008 g/mol, contribution = 2.016 g/mol. Oxygen (O) has 1 atom at 15.999 g/mol, contribution = 15.999 g/mol. Total molar mass = 18.015 g/mol. Percent H = 2.016 / 18.015 x 100 = 11.19%. Percent O = 15.999 / 18.015 x 100 = 88.81%. Sum = 100.00%. These match the defaults pre-filled above.

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Frequently asked questions

What is percent composition by mass?

Percent composition by mass is the fraction of a compound's molar mass contributed by each element, expressed as a percentage. For each element: % = (number of atoms x molar mass of element) divided by molar mass of compound, times 100. The percentages of all elements sum to 100 percent.

What is the percent composition of water H2O?

Water (H2O) has a molar mass of about 18.02 g/mol. Hydrogen contributes 2 atoms x 1.008 g/mol = 2.016 g/mol, giving 11.19 percent. Oxygen contributes 1 atom x 15.999 g/mol = 15.999 g/mol, giving 88.81 percent.

How is percent composition used in chemistry?

Percent composition is used to verify a compound's identity by comparing experimental and theoretical values, to determine the empirical formula from elemental analysis data, and to calculate the mass of a specific element present in a given mass of compound.