How do I calculate the mass percent of an element?

Multiply the element's atomic mass by how many atoms of it are in the formula to get its total mass, divide that by the molar mass of the whole compound, and multiply by 100. In glucose, C6H12O6, carbon contributes 6 times 12.011, which is 72.066 grams per mole out of a molar mass of 180.16, giving about 40.00 percent carbon by mass.

Why should the percentages add up to 100?

Because every atom in the compound belongs to exactly one element, the individual element masses must add up to the total molar mass, so their percentages of that total add up to 100 percent. A sum that is off by more than a small rounding amount means an atom count or atomic mass was entered wrong, which makes this a quick built-in check on your work.

How is percent composition related to empirical formula?

They are inverse problems. Percent composition goes from a known formula to the mass percentages of its elements, while finding an empirical formula goes the other way, from measured mass percentages back to the simplest whole-number ratio of atoms. Chemists use percent composition to predict what an analysis should show, and empirical formula calculations to identify an unknown compound from lab data.

Does this handle parentheses and polyatomic groups?

Yes. The parser understands parentheses and brackets with subscripts, so formulas like calcium nitrate, Ca(NO3)2, or ammonium sulfate, (NH4)2SO4, are expanded correctly before the masses are added. Element symbols must use proper capitalization, a capital letter optionally followed by a lowercase letter, so cobalt is Co while carbon and oxygen are CO.

Percent Composition Calculator, Free

Chemical formula

Molar mass

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Element	Atoms	Atomic mass	Mass	Mass %

How percent composition works

Percent composition by mass tells you what fraction of a compound's weight comes from each element. The recipe is simple: find the molar mass by adding up every atom's atomic mass, then for each element divide its total mass by that molar mass and multiply by 100. Because heavier atoms take a bigger share, oxygen dominates water (88.8%) even though there are twice as many hydrogen atoms. A useful built-in check is that all the percentages must add to 100, so if they do not, an atom count or symbol is wrong. This is the exact reverse of finding an empirical formula, which starts from these percentages and works back to the formula.

Worked example: glucose (C₆H₁₂O₆)

Molar mass: C: 6 × 12.011 = 72.066 H: 12 × 1.008 = 12.096 O: 6 × 15.999 = 95.994 total = 180.156 g/mol Percent composition: %C = 72.066 / 180.156 × 100 = 40.00 % %H = 12.096 / 180.156 × 100 = 6.71 % %O = 95.994 / 180.156 × 100 = 53.28 % sum = 100.00 % ✓

Carbon and oxygen dominate glucose by mass even though hydrogen has the most atoms, because each C and O atom is about 12 and 16 times heavier than a hydrogen atom.

FAQ

What is percent composition by mass?
Each element's share of the compound's total mass, as a percentage: (element mass ÷ molar mass) × 100.

How do I get the mass percent of one element?
Multiply its atomic mass by its atom count, divide by the molar mass, times 100. E.g. C in glucose = 6×12.011 / 180.16 = 40.00%.

Why add up to 100?
Every atom belongs to one element, so the parts must equal the whole. A sum off by more than rounding means a mistake in the formula.

How does it relate to empirical formula?
It is the reverse: percent composition goes formula → percentages; empirical formula goes percentages → formula.

Does it handle parentheses?
Yes — Ca(NO3)2 and (NH4)2SO4 expand correctly. Use proper capitalization (Co is cobalt, CO is carbon + oxygen).

How percent composition works

Worked example: glucose (C6H12O6)

FAQ

Worked example: glucose (C₆H₁₂O₆)