What is an empirical formula?

An empirical formula gives the simplest whole-number ratio of atoms in a compound. For example, glucose (C6H12O6) has the empirical formula CH2O because the ratio C:H:O = 1:2:1. It does not tell you the actual number of atoms, only the ratio.

How does the calculator find the formula from percentages?

It assumes a 100 g sample so percentages become grams directly. Each mass is divided by the element's standard atomic mass (IUPAC 2021) to get moles. Every mole value is then divided by the smallest mole value to produce ratios. If any ratio is close to a simple fraction (1/2, 1/3, 2/3, etc.) the tool multiplies all ratios by the appropriate integer — up to 8 — to clear fractions, then divides by the greatest common divisor to give the final subscripts.

What is the difference between empirical and molecular formula?

The molecular formula is a whole-number multiple of the empirical formula. For example, the empirical formula of hydrogen peroxide is HO and the molecular formula is H2O2 (n=2). You need an independent molar mass measurement (e.g. from mass spectrometry) to determine n.

My percentages do not add up to exactly 100 — is that a problem?

Small rounding errors (within about 1.5%) are normal and the calculator handles them automatically. If the sum differs more than that, check whether one element (often oxygen) was omitted. The tool warns you if the total deviates significantly.

Which elements are supported?

All 83 stable elements plus uranium, thorium, and protactinium — 86 elements in total, with atomic masses from the 2021 IUPAC recommendations. Type the standard chemical symbol (first letter uppercase, rest lowercase): e.g. Fe, Mg, Cl, Br.

How accurate are the atomic masses used?

The calculator uses the 2021 IUPAC standard atomic weights: H=1.008, C=12.011, N=14.007, O=15.999, Fe=55.845, and so on. These match the values in any current chemistry textbook and are accurate to at least three decimal places for all common elements.

Empirical Formula Calculator

An empirical formula calculator that turns raw mass-percentage or gram data into the simplest whole-number atom ratio — and, with a molar mass, into the full molecular formula — showing every step of the working.

How it works

The process follows the standard five-step procedure taught in A-level and university chemistry courses:

Convert to grams. If you are working from percentage composition, assume a 100 g sample so each percentage value is numerically equal to a mass in grams.
Convert grams to moles. Divide each element’s mass by its standard atomic mass (IUPAC 2021). For carbon: n(C) = mass(C) / 12.011 g mol⁻¹.
Normalise to the smallest mole value. Divide every mole count by the smallest value in the set to obtain the mole ratios.
Round to whole numbers. Ratios should be close to integers. When a ratio falls near a simple fraction (e.g. 1.5 → 3/2, 1.333 → 4/3), the tool tests multipliers from 1 to 8 to find the smallest integer factor that clears all fractions simultaneously. It then simplifies by dividing by the greatest common divisor.
Scale to the molecular formula. If you supply the experimental molar mass M, the whole-number multiplier n = round(M / empirical formula mass) is applied to every subscript.

The calculator stores atomic masses for 86 elements to IUPAC 2021 precision.

Worked example — glucose

Combustion analysis of glucose yields:

Element	% by mass
C	40.00 %
H	6.71 %
O	53.29 %

Step 1 — assume 100 g sample: C = 40.00 g, H = 6.71 g, O = 53.29 g

Step 2 — moles:

n(C) = 40.00 / 12.011 = 3.330 mol
n(H) = 6.71 / 1.008 = 6.657 mol
n(O) = 53.29 / 15.999 = 3.331 mol

Step 3 — divide by smallest (3.330): C:H:O = 1.000 : 1.998 : 1.000 ≈ 1 : 2 : 1

Step 4 — empirical formula: CH₂O (formula mass = 30.026 g mol⁻¹)

Step 5 — molecular formula with M = 180.16 g mol⁻¹: n = 180.16 / 30.026 ≈ 6, so the molecular formula is C₆H₁₂O₆ (glucose).

The tool reproduces this result automatically and expands the full working on demand.

Formula note

The key equation is:

moles of element  =  mass (g)  /  atomic mass (g mol⁻¹)

Once mole ratios are found, the empirical formula mass (EFM) is:

EFM  =  Σ  (subscript_i × atomic_mass_i)

The molecular formula multiplier is n = round(M / EFM), where M is the experimentally determined molar mass (from mass spectrometry or other method). If n = 1 the empirical and molecular formulas are identical.