Percent Composition Calculator — Mass Percentage of Each Element

Percent composition tells you what fraction of a compound's mass comes from each element. It's the starting point for empirical formula work, and it's also just genuinely useful to know — like the fact that water is 88.8% oxygen by mass, or that table salt is about 39% sodium. The CalcHub Percent Composition Calculator takes any chemical formula and breaks down the mass contribution of every element.

The Formula

For each element in a compound:

% by mass = (n × atomic mass) / molar mass of compound × 100

Where n is the number of atoms of that element in the formula.

How to Use the Calculator

1. Open CalcHub and navigate to the Percent Composition Calculator.
2. Enter the chemical formula using standard notation (e.g., Ca(OH)₂, C₆H₁₂O₆, Al₂(SO₄)₃).
3. The calculator returns the molar mass plus the mass percentage of each element.

The percentages will always sum to exactly 100% (with possible minor rounding in the last decimal place).

Examples

Water (H₂O):

Element	Atoms	Mass contribution	%
H	2	2 × 1.008 = 2.016	11.2%
O	1	1 × 16.00 = 16.00	88.8%
Total	18.016 g/mol	100%

Calcium carbonate (CaCO₃):

Element	Atoms	Mass contribution	%
Ca	1	40.08	40.0%
C	1	12.01	12.0%
O	3	48.00	48.0%
Total	100.09 g/mol	100%

Real-World Uses

• Fertilizer labels: NPK ratios show percent composition of nitrogen, phosphorus, and potassium
• Mineral analysis: Knowing calcium percentage in CaCO₃ helps calculate how much calcium you can extract from limestone
• Nutritional chemistry: Breaking down amino acid or vitamin composition
• Quality control: Verifying a synthesized compound matches expected elemental ratios (combustion analysis)

Tips

• Hydrated salts: For CuSO₄·5H₂O, include the water of crystallization in your formula. The % composition changes substantially — CuSO₄ alone is 25.5% Cu, but the hydrate is only 20.1% Cu.
• Cross-check with empirical formula: If you experimentally measure percent composition and it matches the calculated value, your compound is likely what you think it is.

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How does percent composition relate to empirical formulas?

Percent composition is the input; empirical formula is the output. If you measure percent composition experimentally (via combustion analysis, for example), you can feed those percentages into the empirical formula calculator to determine the formula of an unknown compound.

What if two compounds have the same percent composition?

Then they're either the same compound or have the same empirical formula but different molecular formulas. Glucose (C₆H₁₂O₆) and formaldehyde (CH₂O) have identical percent compositions (40% C, 6.7% H, 53.3% O) because they share the same CH₂O empirical formula.

How accurate is this for mixtures vs. pure compounds?

This calculator assumes a pure compound. For a mixture of compounds, the measured percent composition would reflect the blend of all components. You'd need additional information (like molecular mass) to deconvolute the individual components.

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Related calculators: Empirical Formula Calculator · Molar Mass Calculator · Stoichiometry Calculator