Empirical Formula Calculator
The empirical formula from percent composition — and the molecular formula if you know the molar mass.
Assume 100 g of the compound, so percentages become grams.
How the empirical formula calculator works
Assume 100 g of the compound, so percentages become grams. Divide each by its atomic mass to get moles, then divide all by the smallest to get a ratio, and scale to whole numbers.
The empirical formula is the simplest ratio; the molecular formula is a whole-number multiple of it. Glucose is CH₂O empirically and C₆H₁₂O₆ molecularly — six times over.
Formula: divide % by atomic mass, then by the smallest result
Worked examples
| Inputs | Empirical formula | Note |
|---|---|---|
| Glucose: 40% C, 6.7% H, 53.3% O | CH2O | CH₂O empirically, C₆H₁₂O₆ molecularly |
| Water | H2O | H₂O |
| Benzene | CH | CH empirically, C₆H₆ molecularly |
FAQFrequently asked questions
What is an empirical formula?
The simplest whole-number ratio of atoms. Glucose's is CH₂O, even though its molecule is C₆H₁₂O₆.
How do I get the molecular formula?
Divide the known molar mass by the empirical formula mass. The whole-number result is the multiplier.
Why assume 100 g?
Because it makes the percentages numerically equal to grams. Any assumed mass gives the same ratio.
What if the ratio is not whole?
Multiply through. A ratio of 1 : 1.5 becomes 2 : 3, which is why this calculator tests multipliers up to twelve.
Can two compounds share an empirical formula?
Yes — formaldehyde, acetic acid and glucose are all CH₂O. The empirical formula alone never identifies a compound.
Where these figures come from
- IUPAC — Standard atomic weights (2021 conventional values) — the molar-mass table
- NIST — CODATA 2018 fundamental physical constants — Avogadro constant, gas constant, speed of light
- NIST Chemistry WebBook — thermochemical data
- CSIRO — Australia's national science agency
Last checked: September 2026. Atomic masses are the IUPAC conventional values; constants are CODATA 2018; equations are the standard textbook forms.