Empirical Formula Calculator
Convert elemental mass or percent composition into the simplest whole-number atom ratio.
Composition inputs
Live result
Composition profile
Calculation steps
| Element | Atomic mass (g/mol) | Input amount | Moles | Normalized ratio | Whole-number subscript |
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How to use the empirical formula calculator
What this calculator does
This calculator turns elemental composition data into an empirical formula: the simplest whole-number ratio of atoms in a compound. It accepts either percent by mass or measured masses, converts each amount to moles using atomic mass, divides all mole amounts by the smallest value, and searches for the smallest reasonable integer multiplier. The result describes composition, not molecular structure, bonding, or the exact number of atoms in one molecule.
When to use it
Use it when checking combustion-analysis or elemental-analysis data, preparing a chemistry lab report, verifying a hand calculation, or testing whether a proposed composition is consistent with a simple formula. It is also useful before converting an empirical formula into a molecular formula when the compound's molar mass is known.
How to calculate
- The calculator opens with a complete demonstration: 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen. The result and a validated example Excel workbook are ready immediately.
- Choose Composition given as: use Percent by mass (%) for percentages or Mass (g) for laboratory masses.
- Replace each Element symbol and its Percentage of mass (%) or Mass (g). Add up to ten elements, or remove unneeded rows.
- Read the Empirical formula, Smallest mole amount, Empirical formula mass, composition chart, and calculation table.
- Select Download Excel to export the current validated model. Reset clears the demonstration and results; export remains unavailable until a complete valid composition is entered again.
Input guide
Composition given as is required and controls the unit used for every amount. Percent mode accepts ordinary decimal percentages such as 40 or 40.0; values must be positive and the total should be close to 100%. Mass mode accepts positive gram values such as 12.5. Because only ratios matter, 12.5 g and 25 g samples with the same relative composition yield the same formula. Do not paste units, commas used as decimal separators, or scientific notation.
Each Element symbol is required and must be a valid chemical symbol such as C, H, O, Na, Cl, or Fe. Symbols are case-normalized, so “cl” becomes “Cl.” Duplicate elements are rejected because their amounts should be combined into one row. Each corresponding amount is required, must be finite and greater than zero, and directly controls the calculated mole amount.
Output guide
Empirical formula is the reduced integer atom ratio. Smallest mole amount is the divisor used to normalize the mole values. Empirical formula mass is the molar mass of one empirical-formula unit. The summary pills show element count, composition total, and the integer scale factor used when normalized ratios are fractional. The table lists atomic mass, input amount, moles, normalized ratio, and final whole-number subscript for every element. The composition chart shows the relative input amounts; it is descriptive and does not change the formula independently of the table.
Worked example
For 40.0% C, 6.7% H, and 53.3% O, assume a 100 g sample. The mole amounts are approximately 40.0/12.011 = 3.3303 mol C, 6.7/1.008 = 6.6468 mol H, and 53.3/15.999 = 3.3315 mol O. Dividing by the smallest gives about 1.0000 : 1.9959 : 1.0004, which rounds within tolerance to 1 : 2 : 1. The empirical formula is therefore CH₂O, and its empirical formula mass is 30.026 g/mol.
Learn more
The IUPAC Gold Book definition of empirical formula explains the formal concept. For element data, consult the IUPAC periodic table. The Chemistry LibreTexts guide to empirical and molecular formulas provides additional worked examples.
How the ratio method works
Masses cannot be compared directly because atoms of different elements have different masses. Dividing each amount by its atomic mass converts the data to moles, which are proportional to numbers of atoms. The smallest mole value becomes the reference. Each mole amount is divided by that value, producing ratios near 1, 2, 3, and so on.
Experimental data rarely produce perfect integers. A ratio near 1.5 suggests multiplying all ratios by 2; a ratio near 1.333 suggests multiplying by 3; and a ratio near 1.25 suggests multiplying by 4. This calculator tests multipliers from 1 through 12 and chooses the first set that falls within a practical tolerance. If no clean ratio is found, it reports the closest result but flags that the data may need review.
Empirical formula versus molecular formula
An empirical formula is always reduced. A molecular formula may be a whole-number multiple of it. For example, CH₂O is the empirical formula for both formaldehyde and glucose, while glucose has the molecular formula C₆H₁₂O₆. To obtain a molecular formula, divide the measured molar mass by the empirical formula mass and multiply every empirical subscript by the resulting whole number.