What Is a Percent Composition Calculator?
Percent composition is the proportion, by mass, that each element contributes to a compound's total molar mass. This calculator sums each element's atomic mass times its subscript, adds them all together for the molar mass, then divides each element's contribution by that total to get its mass percent.
It's a foundational general chemistry skill used to verify a compound's identity, check purity, or set up the reverse calculation of finding an empirical formula from experimental percentages.
How to Read Your Results
Molar Mass
The total mass of one mole of the compound, calculated by summing (atomic mass × subscript) for every element you entered.
Element 1–4 %
Each row shows what fraction of the compound's total mass comes from that element. Leave a row's subscript at 0 if your compound has fewer than four distinct elements — it will be excluded automatically.
Real-World Example
Glucose has the molecular formula C₆H₁₂O₆. Entering carbon (12.011, ×6), hydrogen (1.008, ×12), and oxygen (15.999, ×6) gives the following breakdown:
| Element | Contribution (g/mol) | Percent Composition |
|---|---|---|
| Carbon (C₆) | 72.066 | 40.00% |
| Hydrogen (H₁₂) | 12.096 | 6.71% |
| Oxygen (O₆) | 95.994 | 53.29% |
The molar mass comes out to 180.156 g/mol, and the three percentages sum to 100.00% — a good check that the formula and atomic masses were entered correctly.
Tips for Accurate Results
- Use atomic masses to at least 2 decimal places from a current periodic table for the most accurate percentages.
- Double-check your subscripts against the actual chemical formula — a common source of error.
- Percentages should sum to ~100%; if they're noticeably off, recheck an atomic mass or subscript.
- Use this in reverse with the Empirical Formula Calculator when you only know percentages and need to find the formula.
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Last updated: August 2026 · Reviewed by: Simple Calculator Tools Editorial Team