Percent Composition Calculator
The percent by mass of every element in a compound, worked out from the formula.
The percent composition is each element's share of the total molar mass.
Percent composition of H₂O
88.81% oxygen · 11.19% hydrogen
Molar mass 18.015 g/mol
| Element | Atoms | Atomic mass | Mass | % of total |
|---|---|---|---|---|
| Oxygen | 1 | 15.999 | 15.999 | 88.81% |
| Hydrogen | 2 | 1.008 | 2.016 | 11.19% |
| Total | 3 | 18.015 | 100% |
Working
- O: (1 × 15.999) ÷ 18.015 × 100 = 88.81%
- H: (2 × 1.008) ÷ 18.015 × 100 = 11.19%
Reading the result
The percentages below the answer are calculated from the formula, not measured, so they describe an ideally pure sample. A real analysis will differ slightly, and a large discrepancy usually means the sample is impure or hydrated — water of crystallisation is heavy enough to move every percentage noticeably.
Why it matters
Percent composition is how a mineral assay, a fertiliser grade and a nutrition label are all quoted. It is also the quantity that connects an unknown compound to its formula: measure the composition, and the empirical formula follows.
Related: molar mass calculator, chemical equation balancer, periodic table.
How it works
Percent composition is each element's share of a compound's total mass. It is calculated from the formula, not measured, so it describes an ideally pure sample.
Percent composition by mass
% element = (n × atomic weight ÷ molar mass) × 100 molar mass = Σ (n × atomic weight)
- n
- number of atoms of that element in the formula
- atomic weight
- from the periodic table, g/mol
- molar mass
- total for the whole formula unit
Water, H₂O
- 2 hydrogen at 1.008
- 1 oxygen at 15.999
- Molar mass = 2(1.008) + 15.999 = 18.015
- H: (2 × 1.008 ÷ 18.015) × 100 = 11.19%
- O: (15.999 ÷ 18.015) × 100 = 88.81%
Water is 11.19% hydrogen and 88.81% oxygen by mass — despite having twice as many hydrogen atoms.
Good to know
- The percentages always sum to 100%, which is a free check on your arithmetic.
- More atoms does not mean a larger mass share. Water has twice as many hydrogens as oxygens and is still 89% oxygen by mass.
- A hydrate carries the water in its formula, and it is heavy: CuSO₄·5H₂O is 36% water by mass, which moves every other percentage substantially.
- Percent composition is fixed for a compound. Percent yield is about how a reaction went — they share a word and nothing else.
Related Calculators
Frequently Asked Questions
What is percent composition?
Percent composition is the share of a compound’s total mass contributed by each element. For water, the two hydrogens account for 11.19% of the mass and the oxygen for 88.81%, even though there are twice as many hydrogen atoms — oxygen is simply much heavier.
How do you calculate percent composition?
Work out the molar mass of the compound, then for each element divide its total mass in the formula by that molar mass and multiply by 100. The percentages always add up to 100%, which is a useful check.
What is the difference between percent composition and percent yield?
Percent composition is about what a compound is made of and never changes for a given substance. Percent yield is about how well a reaction went — how much product you actually got against the theoretical maximum. They share a name and nothing else.
Can I go from percent composition back to a formula?
Yes, and that is the empirical formula problem: divide each percentage by the element’s atomic weight, then divide through by the smallest result. Use the empirical formula calculator for that direction.