Percent Composition Calculator
Calculate an element's percent by mass in a chemical compound.
Assumptions used in this calculation
- Standard atomic masses: Assumes standard atomic masses from the periodic table (as of the most recent IUPAC revision) for computing element and molar masses.
About this calculator
Knowing what fraction of a compound's mass comes from a particular element matters for everything from calculating fertilizer nutrient content to determining empirical formulas from experimental mass data, but the calculation, dividing an element's contribution to the formula mass by the total molar mass, requires first working out both of those numbers correctly from the compound's formula. This calculator takes the element's total mass contribution within the formula (its atomic mass times how many atoms of it appear) and the compound's total molar mass, then computes the percent by mass using % = (element mass ÷ total molar mass) × 100. It's built for chemistry students verifying empirical formula problems, checking fertilizer or supplement label claims against a compound's actual formula, or working backward from percent composition data to identify an unknown compound, letting you confirm a hand-calculated ratio or skip the arithmetic on a formula with several elements.
Worked example
Oxygen in water (H₂O)
Result: 88.9%
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