Percent Composition Calculator
Type a chemical formula using capital letters for elements.
This result is an estimate only. Double-check important figures before relying on them.
How it is calculated
- Percent of an element = atoms × atomic mass ÷ molar mass × 100
- Elements are listed from the largest share down
For the molar mass alone, use the molecular weight calculator.
Worked example
Sulfuric acid, H2SO4 (98.072 g/mol):
- Oxygen 65.25%, sulfur 32.69%, hydrogen 2.06%
Why your number may differ
This covers elements 1 to 92. Values depend on the atomic masses used and may differ in the last digit from a textbook.
Frequently asked questions
How is percent composition calculated?
For each element: (atoms × atomic mass) ÷ molar mass × 100.
What do the percentages add up to?
100%, apart from small rounding.
When is it used?
To check a compound's purity or empirical formula, or to work out how much of an element is in a sample.