Percent Composition Calculator

Get molar mass and the mass percent of each element from a formula.

What it does

Percent composition tells you how much of a compound's mass comes from each element. It is a bridge between a chemical formula and a laboratory measurement. A formula gives the atom counts; percent composition gives the mass share you would expect for each element.

The calculation uses the molar mass. For each element, multiply its atomic weight by the number of atoms in the formula. Add those masses to get the molar mass of the compound. Then divide each element's mass by the total and multiply by 100.

Glucose is the default. Its formula is C6H12O6. Carbon contributes 6 x 12.011 = 72.066 g/mol, hydrogen contributes 12 x 1.008 = 12.096 g/mol, and oxygen contributes 6 x 15.999 = 95.994 g/mol. The total is 180.156 g/mol. The mass shares are 40.00% carbon, 6.71% hydrogen, and 53.28% oxygen.

The result is a theoretical composition based on the formula. A real sample can differ because of impurities, moisture, isotopes, or measurement error. The atomic weights used here are standard values, and different periodic tables can change the last digit. The tool supports parentheses and integer subscripts, but not hydrates, charges, or isotopes.

How to use

1. Enter a chemical formula, such as C6H12O6 or Ca(OH)2.
2. Use uppercase for the first letter of each element and lowercase for the second, as in Na or Cl.
3. Add integer subscripts after an element or a closing parenthesis. A missing subscript means 1.
4. Read the molar mass first.
5. Read the mass percent for each element and check that the shares are reasonable.
6. Use the largest share to identify the element that contributes most of the mass.

Formula

M

Molar mass of the compound in grams per mole.

n_i

Number of atoms of element i in the formula.

A_i

Atomic weight of element i in grams per mole.

m_i

Mass of element i in one mole, n_i x A_i.

%i

Mass percent of element i, m_i / M x 100.

Parentheses multiply the counts inside them. In Ca(OH)2, the O and H counts are each multiplied by 2. A subscript after an element applies only to that element. Hydrates written with a dot, such as CuSO4.5H2O, are not supported because the parser cannot tell a hydrate coefficient from a subscript without more rules.

Worked example

Reference table

Limits

• The formula parser supports element symbols, integer subscripts, and parentheses. It does not support hydrates, charges, isotopes, or fractional subscripts.
• The built-in atomic-weight table covers common elements including H, C, N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Fe, Cu, and Zn. Other elements return an error.
• The result is a theoretical mass percent from standard atomic weights, not a laboratory analysis.
• Atomic weights vary slightly by source and by isotopic composition, so the last digit can differ.
• The tool does not check whether the formula is chemically reasonable or charge balanced.

Common mistakes

• Multiplying the atomic weight by the element symbol instead of its subscript.
• Forgetting to multiply every element inside parentheses by the subscript outside.
• Using the mass of the element in the sample instead of the mass in one mole of the formula.
• Dividing by the element mass instead of the compound's molar mass.
• Expecting the percentages to sum to exactly 100.00 after rounding.

Try it yourself

Quick checks. Answers sit tucked under each question.

Questions

Sources

• LibreTexts: Mass percent composition from a chemical formula
• Mississippi State University: Percent composition practice

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