Percent composition tells you exactly what fraction of a compound’s mass comes from each element — an essential first step in stoichiometry, empirical-formula determination, lab report writing, and quality-control calculations. This calculator supports every element from hydrogen (H) to lawrencium (Lr), handles nested brackets, and shows the full arithmetic so you can follow — and check — every step.
How it works
The core formula is straightforward:
% element = (n × Ar) ÷ Mr × 100
where n is the number of atoms of that element in one formula unit, Ar is its standard atomic weight (IUPAC 2021, in g/mol), and Mr is the molar mass of the whole compound — itself the sum of n × Ar for every element.
The calculator tokenises your formula string left-to-right, pushing each element onto a stack and multiplying group contents by any trailing subscript when a bracket closes. This means arbitrarily nested structures like K4[Fe(CN)6] or Ca(H2PO4)2 are handled correctly without any manual expansion on your part.
Atomic masses used are the IUPAC 2021 standard atomic weights (dimensionless but numerically equal to g/mol). For radioactive elements with no stable isotope the calculator uses the mass number of the longest-lived common isotope.
Worked example — glucose (C6H12O6)
Glucose has molar mass Mr = 6(12.011) + 12(1.008) + 6(15.999) = 180.156 g/mol.
| Element | Atoms | Ar (g/mol) | n × Ar (g/mol) | % by mass |
|---|---|---|---|---|
| C | 6 | 12.011 | 72.066 | 40.00% |
| H | 12 | 1.008 | 12.096 | 6.71% |
| O | 6 | 15.999 | 95.994 | 53.29% |
| Total | 180.156 | 100.00% |
Carbon and oxygen together make up nearly 93% of glucose’s mass; hydrogen, despite having 12 atoms, contributes under 7% because its atomic mass is so low.
Why percent composition matters
Empirical formulas from combustion analysis. A classic lab technique burns an organic compound and captures the CO2 and H2O produced. The mass of each product lets you calculate mass percentages of C and H; the remainder is usually O. Dividing each percentage by the element’s atomic mass and finding the smallest whole-number ratio gives the empirical formula. Percent composition is the bridge between experimental data and formula.
Quality control and purity checks. Pharmaceutical and industrial chemists verify a compound’s identity by checking that the measured elemental percentages match the theoretical ones for the expected formula. A discrepancy signals impurity or the wrong compound.
Nutrient and ingredient labelling. The same arithmetic underpins how the nitrogen content of a fertiliser is expressed, why “protein %” on a food label is derived from nitrogen percentage (Kjeldahl method), and how mineral supplements state elemental content (e.g. calcium in calcium carbonate is 40.04%).
Reaction stoichiometry. Knowing the mass fraction of the active element in a reagent lets you convert between “grams of compound used” and “grams (or moles) of element available” — critical when working with impure reagents or hydrates.
Formula reference
The full molar-mass calculation is:
Mr = Σ (ni × Ar,i) for all elements i in the formula
and each element’s percent contribution is:
wi = (ni × Ar,i) ÷ Mr × 100 %
All percentages sum to 100 % (within floating-point rounding). The tool displays the residual rounding error at the bottom of the working panel; it is typically <0.01%.