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Empirical formula calculator

Start with the measured composition of a compound. Compare a candidate formula with the original percentages instead of rounding every mole ratio on its own.

Enter every element

Changing the basis clears the entries. All values must describe the same sample.

Use 2–8 unique element symbols, a space and a positive number. Example: C 40.00. No % signs, compounds or isotope labels. In percent mode, enter values totaling 99.5–100.5% to allow small rounding differences.

Candidate empirical formula

Candidate from your inputs

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Enter all elements to see their mole ratios.

The result is a numerical candidate, not a chemical identification.

The search tests ratio multipliers 1–12, reduces common factors and compares mass percentages. Its 0.1-percentage-point fit limit is a calculator rule, not an experimental uncertainty or proof of a compound's identity.

How composition becomes an empirical formula

An empirical formula describes the simplest whole-number atom ratio. It does not, by itself, tell you the size of a molecule. OpenStax's formula-composition chapter develops the connection between elemental masses, moles and formula subscripts.

  1. Divide each elemental mass by its atomic molar mass.
  2. Divide every mole amount by the smallest one.
  3. Find a common multiplier that makes the ratios close to whole numbers.
  4. Reduce any common factor and check the formula's percentages against the inputs.

Mass percent works the same way: on a notional 100 g basis, 40% carbon corresponds to 40 g of carbon. This tool permits a small total-rounding difference, displays your entered total and normalizes the percentages for its final comparison. It does not fill in a missing element automatically.

Worked example: carbon, hydrogen and oxygen

The example button enters C 40.00%, H 6.71% and O 53.29%. Using the site's atomic weights gives approximately 3.33028 mol C, 6.65675 mol H and 3.33083 mol O on the 100 g basis. Dividing by the smallest gives about 1 : 1.99886 : 1.00017.

The candidate CH₂O has an empirical formula mass of 30.026 g/mol. Its predicted mass percentages are close to the entered values; the table shows the differences rather than claiming the measurements were exact. CH₂O could be the empirical formula of multiple substances. The composition alone does not establish glucose or another particular molecular structure.

Do not round a ratio of 1.5 straight to 2

For 111.69 g Fe and 47.997 g O, dividing by atomic molar mass gives 2 mol Fe and 3 mol O. Normalizing gives 1 : 1.5. Multiply both values by 2 to recover 2 : 3, so the empirical formula is Fe₂O₃. Rounding the oxygen ratio alone would incorrectly suggest FeO₂.

Mass ÷ atomic molar mass = elemental moles

Elemental moles ÷ smallest mole amount = normalized ratio

The grams to moles calculator can check a known final formula. For water associated with a salt and before-and-after mass readings, use the separate hydrate formula calculator.

What the fit rule can and cannot tell you

The app tries integer multipliers from 1 through 12, rounds each scaled ratio and reduces common divisors. A tested candidate qualifies only when each predicted mass percentage differs by no more than 0.1 percentage point from the normalized input. Among qualifying candidates it shows the one with the fewest total atoms. It reports when more than one tested candidate qualifies.

This is a bounded numerical search, not an exhaustive chemical formula finder. Subscripts above 1,000 are outside its scope. It does not check valence, bonding, structure, mixture composition or isotope enrichment. A good numerical fit is not proof that a proposed compound exists.

“No simple fit” preserves the mole table so you can check transcription, units, missing elements and the original experiment's uncertainty. The calculator's cutoff should never replace the uncertainty supplied with your measurements. Even a close fit can be wrong if the inputs describe a mixture.

Empirical and molecular formulas answer different questions

CH₂O gives a 1 : 2 : 1 ratio. C₆H₁₂O₆ describes a molecule with six times those atom counts. To determine a molecular formula you also need independent molecular molar-mass information and an appropriate chemical basis. This tool deliberately stops at a qualified empirical candidate.

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Sources and corrections

Method: OpenStax: empirical and molecular formulas. Atomic weights: CIAAW 2024 abridged values. Central values are used; atomic-weight uncertainties are not propagated.

Reviewed September 17, 2026. Calculations run locally without saved entries. Report an issue to contact@gramstomoles.org.