Empirical Formula Calculator
Estimate an empirical-formula candidate from elemental masses or mass percentages, with mole ratios, a bounded integer search and reported fit errors.
Estimate a simplest-ratio formula candidate from measured elemental masses or mass percentages. The table separates mole conversion from the integer-ratio step and shows how closely the candidate reproduces your composition. Rounded measurements may fit several formulas, so the answer is a candidate rather than a unique identification.
How to use this tool
- Enter each element once, with its mass or mass percentage on a separate line.
- Choose grams or mass percent; include every constituent element.
- Set the permitted percentage-point difference for each element.
- Inspect the candidate, mole ratios and measured-versus-predicted percentages.
Formula
mole amountᵢ = massᵢ / atomic weightᵢ; ratioᵢ = mole amountᵢ / smallest mole amount
The search multiplies ratios by integers 1 through 24, rounds to whole counts, and reduces common factors. Among candidates within tolerance, it chooses the smallest total atom count, then the smallest maximum composition error. Candidates above 1,000 atoms are excluded.
Example
With 111.690 g Fe and 47.997 g O, divide by 55.845 and 15.999 to obtain 2 mol Fe and 3 mol O. The reduced ratio is 2:3, giving Fe2O3. Reconstructing the masses reproduces the input composition.
Frequently asked questions
What is the difference between empirical and molecular formula?
An empirical formula gives the lowest integer atom ratio. CH2O describes the ratio in glucose, but its molecular formula is C6H12O6. A molar-mass measurement is needed to determine the multiplier.
Why can the calculator fail to find a candidate?
The bounded rounding search is not an exhaustive search of every possible formula. No fit may mean insufficient precision, a missing element, a mixture or a ratio outside the supported search. Do not interpret failure as proof that no compound exists.
Do percentages need to sum to 100?
Yes, within 0.1 percentage point for rounding. Accepted values are normalized to 100 before fitting. The tolerance measures percentage points, not relative percent or experimental confidence.
What inputs are supported?
Enter up to twenty distinct elements with CIAAW 2024 standard atomic weights. Amounts must be positive, between 0.000000000001 and one billion. Isotope-enriched samples need different atomic weights; a fit does not establish a pure substance or a chemical structure.
Sources
Last updated: 2026-10-30