Empirical Formula Calculator
The empirical formula of a compound gives the simplest whole-number ratio of atoms. To find it, convert elemental percentages to moles (by dividing by atomic mass), divide all mole values by the smallest, and round to whole numbers. This calculator accepts up to four elements. Enter the mole amounts for each element (or use percent composition directly, since percentages in a 100 g sample equal grams, which then convert to moles by dividing by atomic mass). The calculator outputs the simplified mole ratios.
Empirical formula method
Step 1: moles = mass (g) / atomic mass (g/mol) Step 2: ratio = moles / min(moles) Step 3: round ratios to nearest whole number
If ratios are not close to integers (e.g., 1.5, 2.5), multiply all by the appropriate integer (2 for x.5, 3 for x.33, 4 for x.25) to reach whole numbers.
Worked example: acetic acid
- Composition: C 40.00%, H 6.71%, O 53.29%.
- Moles: C = 40.00/12.011 = 3.330, H = 6.71/1.008 = 6.656, O = 53.29/15.999 = 3.331.
- Divide by minimum (3.330): C = 1.00, H = 1.999 approx 2.00, O = 1.00.
- Empirical formula: CH(2)O (same as formaldehyde; molecular formula of acetic acid is C(2)H(4)O(2) = 2 x CH(2)O).
Frequently asked questions
What is an empirical formula?
An empirical formula gives the simplest whole-number ratio of atoms in a compound. For example, glucose (C6H12O6) has the empirical formula CH2O, which is the simplest ratio of C:H:O = 1:2:1.
How is empirical formula different from molecular formula?
The molecular formula gives the actual number of each atom in one molecule. The empirical formula gives the simplest ratio. The molecular formula is always a whole-number multiple of the empirical formula.
How do I convert percent composition to empirical formula?
Step 1: assume a 100 g sample, so percentages become grams. Step 2: convert grams to moles by dividing by atomic mass. Step 3: divide all moles by the smallest mole value. Step 4: round to whole numbers (if close to a half, multiply all by 2).
When do I multiply the mole ratios?
If after dividing by the smallest mole value you get ratios like 1.5 or 2.5, multiply all ratios by 2. For ratios of 1.33 or 2.33, multiply by 3. For 1.25, multiply by 4.
What inputs does this calculator need?
Enter the number of moles of each element (you can use percent composition values as moles directly in a 100 g sample). The calculator divides all values by the minimum and shows simplified ratios.
Official sources
- IUPAC: IUPAC Gold Book - Empirical formula.
- NIST: NIST Chemistry WebBook.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.