Empirical Formula Calculator
The empirical formula from percent composition or the masses of each element, with every step shown, and the molecular formula when you know the molar mass.
Result…
How it's calculated
Moles of each element = mass (or % of 100 g) ÷ atomic weight Ratio = moles ÷ smallest moles Multiply by 1, 2, 3, 4, 5 or 6 until every ratio is within the tolerance of a whole number n = molar mass ÷ empirical formula mass; molecular formula = empirical formula × n
An empirical formula gives the simplest whole-number ratio of atoms in a compound. From percent composition, imagine 100 g so each percentage becomes grams; divide each mass by the element’s atomic weight to get moles, then divide every mole value by the smallest. Ratios such as 1.5 or 1.33 are not rounding errors: they mean the formula needs doubling or tripling, so the calculator tries multipliers from 1 to 6 and keeps the first that brings every ratio within your tolerance of a whole number. The molecular formula is a whole-number multiple of the empirical one, found by dividing the measured molar mass by the empirical formula mass. Atomic weights come from the CIAAW 2024 abridged table.
Your numbers, step by step
About the Empirical Formula calculator
This empirical formula calculator turns percent composition, or the masses of each element from an analysis, into the simplest whole-number formula of a compound. Type each element symbol with its percentage or mass, and it converts every amount to moles with standard atomic weights, divides by the smallest, and then tries multipliers from 1 to 6 until every ratio sits within your tolerance of a whole number, so ratios like 1.5 or 2.33 are handled properly instead of being rounded away. Add a measured molar mass and it also gives the molecular formula. It is built for chemistry homework and for checking combustion or elemental analysis results. Every stage, from moles to ratios to each multiplier tried, appears in the steps.
How to use it
- Under “I have”, choose “Percent by mass” or “Masses of each element”.
- In “Elements and amounts”, type each symbol and number separated by commas, such as C 40.00, H 6.71, O 53.29. Symbols are case-sensitive.
- Leave the “Whole-number tolerance” at 0.1 for good data, or raise it a little for rough measurements.
- For the molecular formula, set “Molecular formula too” to “From molar mass” and enter the “Molar mass of the compound”.
- Read the formula, then check the steps to see which multiplier was used.
Worked example
A phosphorus oxide is 43.64% P and 56.36% O, with a molar mass of 283.9 g/mol. In 100 g: P 43.64 ÷ 30.974 = 1.40892 mol and O 56.36 ÷ 15.999 = 3.52272 mol. Dividing by the smallest gives P 1 and O 2.500. × 1 leaves O half-way between whole numbers, so try × 2: P 2, O 5.0006, which is within 0.1. The empirical formula is P2O5 (141.943 g/mol). 283.9 ÷ 141.943 = 2.0001, so the molecular formula is P4O10.
Updated
Questions people ask
How do I find the empirical formula from percentages?
Treat the percentages as grams in a 100 g sample, divide each by the element’s atomic weight to get moles, divide all by the smallest, then multiply until every number is close to a whole number.
What do I do if a ratio comes out as 1.5 or 1.33?
Multiply every ratio by the same small whole number: 1.5 needs × 2, 1.33 or 1.67 needs × 3, 1.25 needs × 4. Rounding 1.5 up to 2 would give the wrong formula.
How do I get the molecular formula from the empirical formula?
Divide the measured molar mass by the empirical formula mass and round to a whole number n, then multiply every subscript by n. The molar mass calculator gives the mass of any formula to check against.
What if my percentages don’t add up to 100?
Often one element was not measured directly, usually oxygen, and is found by difference: subtract the others from 100 and add it to the list.
Can an empirical formula tell me the structure of a compound?
No. It only gives the ratio of atoms. Different compounds share the same empirical formula, such as formaldehyde, acetic acid and glucose, which are all CH2O, so you need other data to identify one.