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CHEMISTRY · CALCULATE · UNDERSTAND

Percent Composition Calculator

Find each element's mass percentage from a chemical formula, or convert a known sample mass using that percentage.

High school to introductory general chemistry

Enter your values

Use a decimal point or scientific notation (1e-3). Choose units before calculating.

Use standard element symbols, whole-number subscripts, (), [], an optional hydrate dot, and an optional terminal ion charge.
This unit stays visible and controls the reported mass result.
Try an example

Calculations run on your device. No account, name or email is required.

Your result

Enter your values and press Calculate to see your answer and all five solution steps.

%E = [n(E) × A(E) / M(formula)] × 100

Common Mistakes

  • Using atom counts as percentages: percent composition is based on mass, not number of atoms.
  • Forgetting that a subscript outside parentheses multiplies every atom in that group, such as (OH)2.
  • Omitting waters of hydration after a dot, which changes both molar mass and every percentage.
  • Rounding element contributions before calculating the percentage; retain full calculator precision until the final display.

Assumptions and limitations

  • This is a theoretical composition from a pure formula. It does not measure an impure sample or determine an empirical formula from laboratory data.
  • The calculator supports molecular and ionic formulas with standard symbols, integer subscripts, nested () or [], dot hydrates, and a terminal ion charge. Charge has no material effect on these mass percentages. It does not support equations, isotope labels, fractions, mixtures, polymers, or non-stoichiometric solids.

%E = [n(E) × A(E) / M(formula)] × 100

Learn More

Connect the calculation to your chemistry course and try a related tool.

Molar Mass CalculatorMole CalculatorStoichiometry CalculatorCourse guides and free testsUniversity chemistry tutoring
Scientific references

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