Percent Composition Calculator

At a glance:Percent composition (mass percentage, element mass %) describes 'the percentage of an element's mass in the total mass of a compound' — a direct expression of the chemical formula and the law of definite proportions. The formula is element% = (total mass of the element in one mole of compound ÷ compound molar mass) × 100%. To use it correctly you must understand two quantities: the numerator is 'the total mass of that element in one mole of compound', equal to its atomic weight times the number of that atom in the formula; the denominator is 'the compound molar mass', the sum of all atomic weights by count. Because any pure compound has a fixed formula (law of definite proportions: the mass ratio of elements is constant), percent composition is independent of sample size and is an intrinsic property of the compound. The classic example water H₂O: H atomic weight ≈ 1.008, O ≈ 16.00, molar mass M = 2×1.008 + 16.00 = 18.02 g/mol. Oxygen's total mass is 16.00, so O% = 16.00 / 18.02 × 100% ≈ 88.8%; hydrogen's total mass is 2×1.008 = 2.016, so H% = 2.016 / 18.02 × 100% ≈ 11.2%. The percentages of all elements should sum to 100% (slightly off due to rounding). Carbon dioxide CO₂ (M = 44.01): C% = 12.01/44.01 ≈ 27.3%, O% = 32.00/44.01 ≈ 72.7%. This calculator lets you enter 'element mass in the compound' and 'compound molar mass' directly to get the percentage; when computing element mass, count the atoms (e.g. if the element appears several times). Uses are broad: (1) verifying and deriving formulas — from experimentally measured element percentages you can reverse to the empirical formula: take the percentages as grams in 100 g, divide by atomic weights to get mole ratios, then reduce to the simplest integer ratio; (2) assessing active-ingredient content of fertilisers, minerals, drugs — e.g. nitrogen fertilisers are labelled with nitrogen percent (N%); (3) computing purity, converting composition content, estimating ore grade (metal percent in an ore). Notes: first, use a consistent set of atomic weights (standard periodic table). Second, if the element appears more than once in the molecule, multiply its mass by the count — the most common error source. Third, hydrates (e.g. CuSO₄·5H₂O) must include the water of crystallisation in molar mass and element masses. Fourth, the percentage is theoretically 0–100%; if the input makes the numerator exceed the denominator, the data is wrong and this calculator clamps the result to 100% with a check prompt.