Turn a percent composition or measured masses into the empirical formula, with every step shown: moles of each element, division by the smallest and the whole-number multiplier. Add the molar mass to get the molecular formula too.
Last updated:
Amounts are
One element per line (or comma separated): symbol then number, e.g. Fe 69.94, O 30.06
Combustion analysis often gives oxygen by difference
g/mol
Gives the molecular formula
Enter your values to see the result.
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How to use the Empirical Formula Calculator
Choose percent or grams.
Type each element and its amount, one per line, like C 40.0.
Optionally set a remainder element if one was found by difference.
Enter the molar mass if you also want the molecular formula.
Read the empirical formula and follow the working table.
Empirical formula step by step
Grams: treat percentages as grams in a 100 g sample.
Moles: n = mass ÷ atomic mass for each element.
Ratio: divide every mole value by the smallest one.
Whole numbers: if a ratio is not close to an integer, multiply all of them by 2, 3, 4… until they are.
Molecular formula: n = molar mass ÷ empirical formula mass; multiply all subscripts by n.
Common decimal ratios and their multipliers
Ratio ends in
Multiply by
Example
.5
2
1 : 1.5 → 2 : 3 (Fe₂O₃)
.33 or .67
3
1 : 1.33 → 3 : 4 (Fe₃O₄)
.25 or .75
4
1 : 2.25 → 4 : 9
.2, .4, .6, .8
5
1 : 2.5 is ×2, 1 : 1.2 → 5 : 6
.17 or .83
6
1 : 1.167 → 6 : 7
Worked examples
Composition
Mole ratio
Empirical formula
Molecular formula
C 40.00%, H 6.71%, O 53.29%
1 : 2 : 1
CH₂O
C₆H₁₂O₆ (M = 180.16)
C 92.26%, H 7.74%
1 : 1
CH
C₆H₆ (M = 78.11)
Fe 69.94%, O 30.06%
1 : 1.5
Fe₂O₃
Fe₂O₃
N 30.45%, O 69.55%
1 : 2
NO₂
N₂O₄ (M = 92.01)
Frequently asked questions
What is an empirical formula?
It is the simplest whole-number ratio of atoms in a compound. Glucose has the molecular formula C₆H₁₂O₆ but the empirical formula CH₂O, because 6:12:6 simplifies to 1:2:1.
How do I find the empirical formula from percentages?
Assume a 100 g sample so percentages become grams. Divide each mass by the element's atomic mass to get moles, divide all mole values by the smallest one, then multiply by a small whole number if needed so every ratio is close to an integer.
What if a ratio comes out as 1.5 or 1.33?
Multiply all ratios by the number that makes them whole: ×2 for x.5, ×3 for x.33 or x.67, ×4 for x.25 or x.75. For Fe 69.94% and O 30.06% the ratio is 1 : 1.5, which ×2 gives Fe₂O₃.
How do I get the molecular formula?
Divide the compound's molar mass by the empirical formula mass. The result should be a whole number n; multiply every subscript by n. For CH₂O (30.03 g/mol) and M = 180.16 g/mol, n = 6, giving C₆H₁₂O₆.
What does 'oxygen by difference' mean?
In combustion analysis only C, H (and sometimes N) are measured directly. If the compound also contains oxygen, its percentage is 100% minus the sum of the others. Choose oxygen as the remainder element to do this automatically.
How much rounding is acceptable?
Experimental data usually give ratios within about ±0.1 of a whole number. This calculator accepts ratios within 0.1 and warns you when the best fit is off by more than 0.15.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.