Last updated: June 2026
Use this empirical formula calculator to find the simplest whole-number ratio of atoms in a compound from masses or percentage composition. The molecular formula calculator then uses the empirical formula and relative molecular mass to find the actual molecular formula.
- Choose whether the data are masses or percentage composition.
- Enter each element symbol and its mass or percentage. Leave an entire unused row blank.
- Select Calculate to convert each value into moles and find the simplest whole-number ratio.
- Use the empirical formula in the second calculator and enter the relative molecular mass, Mr.
- The empirical formula mass can be entered manually or left blank for the calculator to determine automatically.
What is an empirical formula?
An empirical formula gives the simplest whole-number ratio of atoms of each element in a compound. It does not always show the actual number of atoms in one molecule.
Ethane has molecular formula C2H6. The simplest carbon-to-hydrogen ratio is 1:3, so its empirical formula is CH3.
Glucose has molecular formula C6H12O6. Dividing every subscript by 6 gives CH2O.
How to find empirical formula
Begin by converting the mass of each element into moles using:
Divide every mole value by the smallest mole value. If the resulting ratio contains values such as 1.5, 1.33 or 1.25, multiply every ratio by the same small whole number until a whole-number ratio is obtained.
Using percentage composition
When percentage composition is given, assume that there are 100 g of the compound. The percentage values can then be treated directly as masses in grams.
For example, 40.0% carbon, 6.7% hydrogen and 53.3% oxygen can be treated as 40.0 g of carbon, 6.7 g of hydrogen and 53.3 g of oxygen.
Empirical formula worked examples
Example 1: Percentage composition
A compound contains 40.0% carbon, 6.7% hydrogen and 53.3% oxygen.
Dividing by the smallest amount gives C:H:O = 1:2:1, so the empirical formula is CH2O.
Example 2: Mass data
A compound contains 2.4 g of carbon and 0.6 g of hydrogen.
Dividing by 0.200 gives C:H = 1:3, so the empirical formula is CH3.
Molecular Formula Calculator
The molecular formula gives the actual number of atoms of each element in one molecule. To find it, divide the relative molecular mass by the empirical formula mass:
The multiplier should be a positive whole number. Multiply every subscript in the empirical formula by this value.
How to find molecular formula from empirical formula
For CH2O, the empirical formula mass is:
If Mr = 180:
Multiplying every subscript by 6 gives C6H12O6.
Common exam traps
Frequently Asked Questions
What does an empirical formula calculator calculate?
It calculates the simplest whole-number ratio of atoms in a compound from masses or percentage composition.
What does a molecular formula calculator calculate?
It uses the empirical formula and relative molecular mass to find the actual molecular formula.
Can percentages be used instead of masses?
Yes. Assume a 100 g sample so that each percentage becomes a mass in grams.
Why must mass be converted into moles?
Chemical formulae compare numbers of atoms rather than masses. Moles allow the numbers of atoms of different elements to be compared.
Why might the multiplier not be a whole number?
This usually means that the empirical formula, empirical formula mass or relative molecular mass has been entered incorrectly or rounded too much.