This percent composition calculator breaks a compound into its elemental recipe: type a chemical formula and get each element’s percentage by mass, alongside the compound’s molar mass.
| Element | Atoms | Mass contribution (g/mol) | % by mass |
|---|
What Percent Composition Tells You
Percent composition is a compound’s ingredient list by weight — which elements, in what proportions. It is how chemists verify a synthesized compound matches its formula, how fertilizer bags get their N-P-K numbers, and the necessary first step toward an empirical formula (run the percentages backwards and the formula falls out). The math is a direct extension of our Molar Mass Calculator: each element contributes atoms × atomic weight; its percentage is its slice of the total. Part of our Chemistry Calculators collection.
Worked Example (Verified)
Water, H₂O (molar mass 18.015 g/mol):
- Hydrogen: 2 × 1.008 = 2.016 g → 2.016 ÷ 18.015 = 11.19%
- Oxygen: 1 × 15.999 = 15.999 g → 88.81%
- By count water is two-thirds hydrogen; by mass it is nearly nine-tenths oxygen — the recurring lesson that atoms have wildly different weights, and percent composition speaks in mass.
- Try glucose, C₆H₁₂O₆: 40.00% C, 6.71% H, 53.29% O — the percentages analytical labs check against combustion analysis.
Reading and Using the Results
- Purity checks: measured composition materially off the theoretical values signals impurities or a wrong identity — the original quality-control test, still run daily.
- Hydrates count fully: in CuSO₄·5H₂O the waters are part of the crystal and part of the mass (the five waters alone are ~36% of it) — enter hydrates with parentheses as the input label suggests.
- Fertilizer logic: urea, CO(NH₂)₂, is 46.6% nitrogen by mass — the highest of common solid fertilizers, which is why it dominates world nitrogen trade. The calculator verifies it in seconds.
Related Tools
- Molar Mass Calculator — the totals behind the percentages
- Empirical Formula Calculator — percentages back into formulas
- Chemistry Calculators — the full collection
Frequently Asked Questions
How do you calculate percent composition?
For each element: (atoms × atomic mass) ÷ compound’s molar mass × 100. Water’s hydrogen: (2 × 1.008) ÷ 18.015 = 11.19%.
What is the percent composition of glucose?
C₆H₁₂O₆ (180.16 g/mol): carbon 40.00%, hydrogen 6.71%, oxygen 53.29%. Enter it above to see the per-element table.
Why don’t the percentages always add to exactly 100%?
Rounding — each element is rounded to two decimals for display, so totals of 99.99% or 100.01% appear. The underlying sum is exact.
What is the difference between percent composition and empirical formula?
Composition goes formula → percentages; empirical formula goes percentages (or masses) → simplest formula. They are inverse operations, and chemistry courses teach them back-to-back for that reason.
Can this handle parentheses and hydrates?
Yes — nested parentheses with multipliers parse correctly (Ca(OH)₂, Al₂(SO₄)₃). For hydrates like CuSO₄·5H₂O, write the water as a parenthesized group: CuSO4(H2O)5.
Is this percent composition calculator free?
Yes — free, instant, no sign-up.