Mass Percent Composition Calculator
Mass percent expresses how much of a mixture or compound is made up of a particular component, by mass. It is the standard way to report composition in chemistry, metallurgy and food labelling. This calculator takes the mass of the component and the total mass, and returns the mass percent, the mass fraction, and the equivalent concentration in parts per million. To find the mass percent of an element in a compound, enter the element's total atomic-mass contribution as the component and the compound's molar mass as the total.
Mass percent formula
mass fraction = mass component / total mass
mass percent = mass fraction * 100
ppm = mass fraction * 1,000,000
remaining = 100 - mass percent
Both masses must use the same unit. For element composition in a compound, use the element's atomic-mass total as the component mass and the compound's molar mass as the total mass.
Worked example: hydrogen in water
- Two hydrogen atoms contribute 2.016 g per mol.
- Water's molar mass is 18.015 g per mol.
- Mass fraction = 2.016 / 18.015 = 0.1119.
- Mass percent = 11.19 percent hydrogen by mass.
- The remaining 88.81 percent is oxygen.
Mass percent: frequently asked questions
What is mass percent?
Mass percent (also called percent by mass or weight percent) is the mass of one component divided by the total mass of the mixture or compound, multiplied by 100. It tells you what fraction of the total mass a given component contributes.
How do I find the mass percent of an element in a compound?
Divide the total mass contributed by that element by the molar mass of the whole compound, then multiply by 100. For example, in water the two hydrogen atoms contribute about 2.016 g per mol out of 18.015 g per mol, giving about 11.19 percent hydrogen.
What is the difference between mass percent and mole percent?
Mass percent is based on mass, while mole percent is based on the number of moles. They differ whenever components have different molar masses. This calculator uses mass, so enter masses (or molar-mass contributions), not mole counts.
Can mass percents add up to more or less than 100?
For a complete mixture the mass percents of all components must sum to exactly 100 percent, because the parts make up the whole. If your figures do not sum to 100, a component is missing or a mass is wrong.
Official sources
- NIST: atomic weights and isotopic compositions.
- IUPAC Gold Book: mass fraction and percent definitions.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.