π¬ Empirical Formula Calculator
Enter element percentages (must sum to β€100%). Add up to 6 elements.
Empirical Formula
How to Use This Calculator
Enter the mass percentage of each element in your compound. The percentages should sum to 100% (or slightly less if you know the remainder is oxygen). You need at least two elements. The tool converts percentages to mole ratios and finds the smallest whole-number formula automatically.
Type the element symbol (exactly as in the periodic table: C, H, O, N, Fe, Cl) in the first column. Capitalisation matters.
Enter the mass percentage for each element. For a compound that is 40.00% C, 6.67% H, and 53.33% O, enter those three values.
Click "Add element" if you need more than three rows. The calculator accepts up to 6 elements.
Click Calculate Formula. The result shows the empirical formula and a breakdown of the mole ratio for each element.
The Method Step by Step
The key step is dividing by the smallest mole value. This sets the element with the fewest moles to a ratio of 1.000 and scales all others relative to it. When you get a ratio like 1.50, multiply everything by 2. When you get 1.33, multiply by 3. When you get 1.25, multiply by 4.
Worked Examples
Where This Calculation Comes Up
The empirical formula is the first result you get from combustion analysis, the standard method for identifying organic compounds in the lab. You burn a precisely weighed sample in excess oxygen and measure the mass of COβ and HβO produced. From those masses you calculate the grams of C and H in the original sample, find oxygen by subtraction, then convert to mole ratios to get the empirical formula. A sample producing 2.64 g COβ and 0.72 g HβO from 1.00 g sample contains 0.72 g C (27.3%), 0.08 g H (8.0%), and by subtraction 0.64 g O (64.7%), giving the empirical formula CHOβ, which is formic acid.
The empirical formula also appears on standardised tests including AP Chemistry, the MCAT, and UK A-level papers, usually as part of a two-step problem where you first find the empirical formula from composition data, then use the molar mass to find the molecular formula. Knowing whether to multiply the ratio by 2, 3, or 4 is a skill that only comes from practice, and this calculator lets you check your work before the exam.
Frequently Asked Questions
What is an empirical formula?
The empirical formula is the simplest whole-number ratio of atoms in a compound. For example, glucose CβHββOβ has empirical formula CHβO.
How do you calculate the empirical formula?
1) Convert % to grams (assume 100 g). 2) Convert grams to moles. 3) Divide by the smallest moles. 4) Round to nearest whole number (multiply if needed).
When must I multiply the ratios?
When ratios are close to X.5, X.33, or X.25, multiply all by 2, 3, or 4 respectively to get whole numbers.
What if percentages don't sum to 100%?
The remainder is assumed to be oxygen, or there may be a measurement error. This calculator auto-adds oxygen if totals less than 100%.
Example: 40% C, 6.67% H, 53.33% O?
C: 40/12=3.33; H: 6.67/1.008=6.62; O: 53.33/16=3.33. Divide by 3.33 β C:1, H:2, O:1 β CHβO (formaldehyde)