Percent Composition Calculator
Find the mass percent of an element in a compound.
Results update as you type.
How it works
Percent composition tells you what fraction of a compound's mass comes from a particular element, and this calculator computes that mass percent directly. You enter the mass of the element in question and the total mass of the compound, and it returns the percentage. For a pure compound this is usually done with molar masses: the element's contribution is the number of its atoms multiplied by its atomic mass, and the total is the compound's molar mass. For example, the two hydrogen atoms in water contribute about 2.016 grams per mole out of water's 18.02 grams per mole, giving hydrogen a mass percent of roughly 11.2 percent, with oxygen making up the rest. Percent composition is a fixed property of any pure compound, reflecting the law of definite proportions, and it is used to verify a compound's identity, to determine empirical formulas from experimental data, and to work out how much of a desired element a raw material contains, which matters in metallurgy, agriculture, and nutrition labelling. Chemistry students calculate it routinely, and it also appears whenever purity or elemental content must be quantified. Enter the element mass and the total mass to find the mass percent.
mass percent = element mass / total mass × 100
Percent composition by mass is the mass contributed by one element divided by the total mass of the compound, multiplied by 100. Using molar masses, the element mass is the number of that element's atoms times its atomic mass, and the total is the compound's molar mass. The result tells you what fraction of the compound's weight comes from that particular element, which is fixed for any pure compound.
Source:Chemistry (law of definite proportions)
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Worked example
How to use this calculator
- 1
Enter your values
Enter element mass and total compound mass.
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Adjust the options
Fine-tune any options to match your situation.
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Read your result
Mass percent updates instantly as you type — no button to press.
Frequently asked questions
What is percent composition?
Percent composition, or mass percent, is the percentage of a compound's total mass that comes from each element it contains. It is calculated by dividing the mass of an element by the total mass of the compound and multiplying by 100. Because pure compounds have fixed formulas, their percent composition is constant, a consequence of the law of definite proportions.
How do I calculate mass percent?
Divide the mass contributed by the element by the total mass of the compound, then multiply by 100. Using molar masses, multiply the number of atoms of the element by its atomic mass to get its contribution, and use the compound's molar mass as the total. The calculator handles the division and percentage once you supply the element mass and total mass.
Why is percent composition useful?
It lets you verify a compound's identity, since each pure compound has a characteristic set of mass percentages. It is also the basis for determining empirical formulas from experimental analysis, and it tells you how much of a valuable element a material contains, which matters in mining, fertiliser formulation, and food nutrition labels. It turns a chemical formula into practical mass information.
Does percent composition change with sample size?
No. Percent composition is an intensive property, meaning it does not depend on how much of the compound you have. A gram or a kilogram of pure water has exactly the same 11.19 percent hydrogen by mass. This is why you can calculate it from molar masses for one mole and apply the result to any amount of the pure compound.
How is it used to find empirical formulas?
Given the percent composition from an experiment, you convert each element's percentage to grams, then to moles using atomic masses, and finally reduce the moles to the smallest whole-number ratio. That ratio is the empirical formula. So percent composition and empirical formulas are two sides of the same relationship, one derived from the other through the mole concept.