Percent Composition by Mass: How to Calculate It and Why It Matters

Percent composition tells you what fraction of a compound's mass comes from each element. It's a direct way to compare compounds, check the identity of an unknown sample, and understand product labels that list nutrients or active ingredients.

The formula

mass % of element = (atoms of element × atomic weight ÷ molar mass of compound) × 100

Example: water, H₂O

Molar mass: 2(1.008) + 15.999 = 18.015 g/mol
Hydrogen: 2.016 ÷ 18.015 = 11.19%
Oxygen: 15.999 ÷ 18.015 = 88.81%

The percentages always add up to 100% (within rounding), which is a quick way to check your work.

Example: glucose, C₆H₁₂O₆

Element	Mass in formula	Mass %
C	6 × 12.011 = 72.066	40.00%
H	12 × 1.008 = 12.096	6.71%
O	6 × 15.999 = 95.994	53.28%
Total 180.156		100%

The molecular weight calculator shows this table automatically for any formula.

Percent of a group, not just an element

The same method works for a group of atoms. In copper(II) sulfate pentahydrate, CuSO₄·5H₂O (249.68 g/mol), the water of hydration accounts for 5 × 18.015 = 90.08 g, or 36.1% of the mass. Heating the crystals to drive off the water and weighing the remains is a classic lab that tests exactly this; see molar mass of hydrates.

Real-world uses

Fertilizer: urea, CO(NH₂)₂, is 46.6% nitrogen by mass, which is why bags are labelled 46-0-0.
Nutrition: sodium chloride is 39.3% sodium, so 1 g of table salt contains about 393 mg of sodium.
Metallurgy: the iron ore hematite, Fe₂O₃, is 69.9% iron by mass, a figure used to judge ore value.
Identifying unknowns: combustion analysis gives percent composition, which leads to the empirical formula; see empirical and molecular formulas.

Going from percent to grams

Multiply the sample mass by the percentage. How much iron is in 500 g of pure Fe₂O₃? 500 × 0.699 = 349.5 g. This is a simple proportion; the proportion calculator works too.