Percent Composition Calculator
By Santaji GadepatilinUpdated October 27, 2026
Percent composition shows what share of a compound's mass comes from each element. It is used to identify substances and check purity.
How to use the percent composition calculator
Enter how many atoms of the element are in the formula, the element’s atomic mass and the compound’s molar mass. Repeat for each element to get the full composition.
Percent composition formula
% element = (atoms × atomic mass) ÷ molar mass × 100
Worked example — water (H₂O)
- Hydrogen: 2 × 1.008 ÷ 18.015 = 11.2%
- Oxygen: 1 × 15.999 ÷ 18.015 = 88.8%
Percent composition examples
| Compound | Element | Percent by mass |
|---|---|---|
| H₂O | O | 88.81% |
| CO₂ | C | 27.29% |
| NaCl | Na | 39.34% |
| CaCO₃ | Ca | 40.04% |
| Fe₂O₃ | Fe | 69.94% |
| C₆H₁₂O₆ | C | 40.00% |
| NH₄NO₃ | N | 35.00% |
Empirical formula from percent composition
- Assume 100 g of compound so percentages become grams.
- Divide each by its atomic mass to get moles.
- Divide all by the smallest number of moles.
- Multiply to get whole numbers if needed.
Example: 40.0% C, 6.7% H, 53.3% O → 3.33 mol C, 6.65 mol H, 3.33 mol O → ratio 1 : 2 : 1 → empirical formula CH₂O.
Fertiliser labels
Fertiliser grades such as 46-0-0 (urea) give nitrogen, phosphate (as P₂O₅) and potash (as K₂O) percentages. Urea (CO(NH₂)₂, 60.06 g/mol) contains 2 × 14.007 ÷ 60.06 = 46.6% nitrogen.
Hydrates
Water of crystallisation counts in the mass. Copper sulphate pentahydrate is 36.1% water by mass.
More examples
| Compound | Element | % by mass |
|---|---|---|
| CO₂ (44.01) | C | 27.3% |
| NaCl (58.44) | Na | 39.3% |
| C₆H₁₂O₆ (180.16) | C | 40.0% |
| Fe₂O₃ (159.69) | Fe | 69.9% |
Frequently asked questions
How do I find the percent composition of an element?
Multiply the number of atoms by the atomic mass, divide by the compound's molar mass, and multiply by 100.
What is the percent of oxygen in water?
16.00 ÷ 18.015 = 88.8%. Hydrogen makes up the other 11.2%.
Do the percentages add to 100?
Yes — the percentages of all elements in a compound total 100% (within rounding).
What percent of water is oxygen?
88.8% by mass.
How do I find the empirical formula from percentages?
Convert percentages to moles (assuming 100 g), divide by the smallest, and round to whole-number ratios.