Percent composition is a calculation that determines the percentage by mass of each element in a compound. It is calculated by taking the mass of one element, dividing it by the total mass of the compound, and multiplying by 100. For example, water (H2O) has a percent composition of 89.9% oxygen because oxygen has a mass of 16.0 grams in water's total mass of 18.0 grams. Percent composition provides information about the elemental makeup of compounds.