This empirical formula calculator turns percent composition (or grams of each element) into the simplest whole-number formula of a compound. Enter up to four elements with their percentages and get the mole ratios and the empirical formula, step by step.
What Is an Empirical Formula?
The empirical formula is the simplest whole-number ratio of atoms in a compound. Glucose’s molecular formula is C₆H₁₂O₆, but its empirical formula is CH₂O — the same 1:2:1 ratio, reduced. Elemental analysis of an unknown compound gives percentages first, so the empirical formula is the first identity a new compound gets. If the molar mass is also known, multiplying the empirical formula up to match it gives the molecular formula — convert with our Molar Mass Calculator. This calculator is part of our Chemistry Calculators collection.
The Method (Worked Example, Verified)
A compound is 40.0% carbon, 6.7% hydrogen and 53.3% oxygen. Assume 100 g of compound:
- Moles: C = 40.0 ÷ 12.01 = 3.33; H = 6.7 ÷ 1.008 = 6.65; O = 53.3 ÷ 16.00 = 3.33
- Divide by the smallest (3.33): C = 1, H = 2, O = 1
- Empirical formula: CH₂O — the building block of sugars (formaldehyde and acetic acid share it too, which is why the molar mass step matters)
If ratios come out like 1 : 1.5 : 1, multiply everything by 2 to clear the fraction — the calculator tries multipliers 2, 3, 4, 5, 6 and 8 automatically.
Percent Composition ⇄ Empirical Formula
The two are inverses: percent composition of a known formula comes from its molar mass breakdown, and the empirical formula comes from reversing that arithmetic on measured percentages. Combustion analysis — burning a sample and weighing the CO₂ and H₂O produced — is the classic laboratory source of those percentages for organic compounds.
Related Tools
- Molar Mass Calculator — from empirical to molecular formula
- Percent Yield Calculator
- Chemistry Calculators — the full collection
Frequently Asked Questions
What is the difference between empirical and molecular formulas?
The molecular formula is the actual atom count in one molecule; the empirical formula is that count divided down to the smallest whole-number ratio. They are identical when the molecule is already in lowest terms (like H₂O).
How do I go from empirical to molecular formula?
Divide the compound’s molar mass by the empirical formula’s mass — the result (a whole number) is the multiplier for every subscript. CH₂O has mass 30; a molar mass of 180 → ×6 → C₆H₁₂O₆.
My ratios are 1 : 1.33 : 1. What now?
Multiply all ratios by 3 to get 3 : 4 : 3. The calculator tests small multipliers automatically when simple rounding fails.
Can I enter grams instead of percentages?
Yes — grams from any sample size work identically, because only the ratios matter. Percentages are just grams per 100 g of compound.
Which elements does the calculator know?
The common elements of introductory and organic chemistry (H through Pb essentials, including C, H, N, O, S, P, Cl, Fe, Cu, Zn, Na, K, Ca and more).
Is this empirical formula calculator free?
Yes — free, instant, no sign-up.