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Empirical Formula Calculator
Derive an empirical formula from percentage composition, and find the molecular formula when a molar mass is known.
🔒 This tool runs entirely in your browser. Your files are never uploaded to a server.
Math & Science
Empirical Formula Calculator
Frontend preview — no upload or external service.
Formula
C 40.0%, H 6.7%, O 53.3% gives mole ratios 3.33 : 6.65 : 3.33, which simplifies to CH₂O. With a molar mass of 180.16, the molecular formula is C₆H₁₂O₆.
How the Empirical Formula Calculator works
- Enter the percentage composition, which should total close to 100%.
- Read the mole ratios — the step where percentages are divided by atomic masses is where the chemistry happens.
- Add the molar mass if you know it, to scale the empirical formula up to the molecular one.
The method
Divide each percentage by the element's atomic mass to get relative moles, then divide all results by the smallest to get a whole-number ratio.
40.0/12.01 = 3.33, 6.7/1.008 = 6.65, 53.3/16.00 = 3.33 → divide by 3.33 → 1 : 2 : 1
The molecular formula is a whole-number multiple of the empirical one: 180.16 ÷ 30.03 = 6, so CH₂O becomes C₆H₁₂O₆.
FAQ
What is the difference between empirical and molecular formulas?
The empirical formula is the simplest whole-number ratio of atoms; the molecular formula is the actual count in one molecule. Glucose, formaldehyde and acetic acid all reduce toward CH₂O-type ratios, which is exactly why the molar mass is needed.
What if the ratios aren't whole numbers?
Multiply everything by a small integer. A ratio ending in .5 needs doubling, .33 needs tripling. Rounding 2.5 to 3 is the classic error and gives the wrong formula.
Why doesn't my percentage total 100?
Rounding in the source data, or an element not reported — often oxygen, which is obtained by difference in combustion analysis. A total a little under 100% is normal; well under suggests a missing element.
How we compare
| Feature | Online Tool Store | A graphing calculator | A stats package |
|---|---|---|---|
| Shows mole ratio working | ✓ | Answer only | ✓ |
| Molecular formula from molar mass | ✓ | Manual | Yes |
| No account | ✓ | ✓ | ✗ |
| Works offline once loaded | ✓ | ✓ | ✗ |
Empirical Formula Calculator shows the mole ratios before simplifying, because rounding a 2.5 ratio to 3 is where these calculations usually go wrong.