📊 Percent Composition Calculator
Enter a chemical formula to get the mass percent of each element — each element’s mass ÷ the molar mass, shown as a clear breakdown.
| Element | Atoms | Atomic wt | Mass | Mass % |
|---|---|---|---|---|
| O Oxygen | 6 | 15.999 | 95.994 | 53.28% |
| C Carbon | 6 | 12.011 | 72.066 | 40.00% |
| H Hydrogen | 12 | 1.008 | 12.096 | 6.71% |
| Total | 180.156 | 100% | ||
IUPAC/CIAAW standard atomic weights. Parser handles nested groups, hydrates (· / . / *) and two-letter symbols. 🔒 Computed in your browser.
How the percent composition calculator works
The molar mass is computed by parsing the formula and summing atomic weights; each element’s total mass (atomic weight × count) is then divided by the molar mass and expressed as a percentage. The percentages sum to 100%.
Percent composition is just a view of the molar-mass calculation — the same exact parse, presented as the mass fraction each element contributes.
Frequently asked questions
How do I calculate percent composition?
For each element: (mass of that element in the formula ÷ molar mass) × 100. In water, H is (2×1.008 ÷ 18.015) × 100 = 11.19% and O is 88.81%.
Do the percentages add to 100?
Yes — every atom’s mass contribution is counted, so the element percentages sum to 100% (aside from tiny rounding in the display).
What is percent composition used for?
To find how much of an element a compound contains by mass — useful for empirical formula work, purity checks, and comparing fertilisers or minerals by nutrient content.
Does it handle hydrates?
Yes — the water of hydration is included, so the percentages reflect the full hydrated formula.
Is my data uploaded?
No — the calculation is entirely in-browser.