Empirical Formula Calculator — From Percent Composition to Simplest Formula

The empirical formula is the simplest version of a compound's formula — it shows the smallest whole-number ratio of atoms. Glucose (C₆H₁₂O₆) has an empirical formula of CH₂O. Finding it from percent composition data involves division and ratio reduction that's tedious to do precisely by hand. The CalcHub Empirical Formula Calculator handles the full process, including the mole ratio reduction step that often produces awkward decimals.

The Process

1. Convert percent composition to grams (assume 100 g sample)
2. Convert grams to moles using each element's atomic mass
3. Divide all moles by the smallest mole value
4. Round to nearest whole number (or multiply to clear fractions)

The empirical formula uses those whole numbers as subscripts.

How to Use the Calculator

1. Open CalcHub and select the Empirical Formula Calculator.
2. Enter each element and its percentage by mass.
3. Percentages should sum to 100% (or close to it — experimental data has rounding).
4. Click Calculate — the tool returns the empirical formula and the mole ratio at each step.

If you also know the molecular mass of the compound, you can enter it to get the molecular formula (which may be a multiple of the empirical formula).

Worked Example

A compound is 40.0% C, 6.7% H, and 53.3% O by mass. Find the empirical formula.

Element	% Mass	Grams (per 100g)	Moles	Divide by smallest	Ratio
C	40.0	40.0 g	40.0 / 12.01 = 3.33	3.33 / 3.33 = 1.00	1
H	6.7	6.7 g	6.7 / 1.008 = 6.65	6.65 / 3.33 = 2.00	2
O	53.3	53.3 g	53.3 / 16.00 = 3.33	3.33 / 3.33 = 1.00	1

Empirical formula: CH₂O

If the molecular mass is given as 180 g/mol:
180 / 30 (mass of CH₂O) = 6 → Molecular formula: C₆H₁₂O₆ (glucose)

Handling Non-Integer Ratios

Sometimes the division gives something like 1.5 instead of a whole number. Multiply everything by 2. If you get ~1.33, multiply by 3. The calculator does this automatically, but understanding why helps:

Decimal	Multiply by
x.5	2
x.33 or x.67	3
x.25 or x.75	4

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What's the difference between empirical and molecular formulas?

The empirical formula shows the simplest ratio; the molecular formula shows the actual number of atoms in one molecule. They may be the same (H₂O = H₂O) or different (empirical CH₂O, molecular C₆H₁₂O₆). The molecular formula is always a whole-number multiple of the empirical formula.

What if my percentages don't add up to exactly 100%?

Experimental data always has some rounding error. The calculator is tolerant of small deviations. If your percentages are significantly off (say, total 98% instead of 100%), check whether there's an unaccounted element — it might be oxygen inferred from the remainder.

Can I input mass in grams instead of percent?

Yes. The calculator accepts either percent composition or absolute masses of each element in a sample. The math is identical — you're just skipping the "assume 100 g" step.

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Related calculators: Percent Composition Calculator · Molar Mass Calculator · Stoichiometry Calculator