What is an empirical formula?

It is the simplest whole-number ratio of elements in a compound.

How do I calculate empirical formula from percentages?

Assume a 100 g sample, convert each percentage to grams, convert grams to moles, divide by the smallest mole amount, and multiply to whole numbers.

Why do we divide by the smallest mole value?

That turns all mole amounts into relative ratios compared with the smallest amount.

What if I get 1.5 in the ratio?

Multiply all ratios by 2 to get whole numbers.

Is empirical formula the same as molecular formula?

Not always. Molecular formula may be a multiple of the empirical formula.

Can this calculator use grams instead of percent?

Yes. Enter masses directly; the mole-ratio method is the same.

Why are atomic masses needed?

Mass percentages must be converted to moles because formulas represent atom ratios, not mass ratios.

Education

Empirical Formula Calculator

Calculate an empirical formula from percent composition or element masses. Enter element symbols and values to get mole ratios, the whole-number multiplier, empirical formula mass, and optional molecular formula.

Enter element symbols and percent or mass values. Percent composition works like a 100 g sample.

Element	Percent or mass

Optional molecular molar mass (g/mol)

Empirical formula

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Mole ratios—

Multiplier used—

Empirical formula mass—

Molecular formula—

How empirical formula calculation works

An empirical formula gives the simplest whole-number ratio of atoms in a compound. OpenStax explains how to determine empirical and molecular formulas in its empirical and molecular formulas section. The standard method is to convert each element's mass to moles, divide by the smallest mole amount, then multiply to reach whole numbers.

mass → moles = mass ÷ atomic mass mole ratio = each mole amount ÷ smallest mole amount empirical formula = simplest whole-number atom ratio

Worked example

A compound is 40.0% C, 6.7% H, and 53.3% O. Assume 100 g: C = 40.0 g, H = 6.7 g, O = 53.3 g C moles = 40.0 ÷ 12.011 ≈ 3.33 H moles = 6.7 ÷ 1.008 ≈ 6.65 O moles = 53.3 ÷ 15.999 ≈ 3.33 Divide by 3.33: C₁H₂O₁ Empirical formula = CH₂O

Empirical formula vs. molecular formula

The empirical formula is the simplest ratio. The molecular formula shows the actual number of atoms in a molecule. If you know the compound's molar mass, divide it by the empirical formula mass to find the whole-number multiplier.

Common questions

It is the simplest whole-number ratio of elements in a compound.
Assume a 100 g sample, convert each percentage to grams, convert grams to moles, divide by the smallest mole amount, and multiply to whole numbers.
That turns all mole amounts into relative ratios compared with the smallest amount.
Multiply all ratios by 2 to get whole numbers.
Multiply all ratios by 3.
Not always. Molecular formula may be a multiple of the empirical formula.
Yes. Enter masses directly; the mole-ratio method is the same.
Mass percentages must be converted to moles because formulas represent atom ratios, not mass ratios.