Percent Composition Calculator — Free Chemistry Calculator

Enter a chemical formula — get the mass percentage of each element instantly, with visual progress bars

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What Is Percent Composition?

Percent Composition Defined

Percent composition is the mass percentage of each element in a compound. It shows how much each element contributes to the total mass — essential for determining empirical formulas, purity analysis, and stoichiometric calculations in chemistry.

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How It Is Calculated

For each element: multiply its atomic count by its IUPAC atomic weight to get its mass contribution. Divide by total molar mass and multiply by 100. The sum of all percentages should equal 100% (within rounding tolerance).

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IUPAC Atomic Weights

This tool uses IUPAC 2021 standard atomic weights (Commission on Isotopic Abundances and Atomic Weights), verified against NIST Standard Reference Database 144. Values reflect the weighted average of naturally occurring isotopes.

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Real-World Applications

Percent composition is used to: verify the purity of a chemical sample, determine empirical formulas from experimental data, calculate nutrient content in fertilizers, and check the composition of alloys and minerals.

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Frequently Asked Questions

What is percent composition?
Percent composition is the percentage by mass of each element in a chemical compound. It tells you, for example, that water (H2O) is 11.2% hydrogen and 88.8% oxygen by mass.
How do I enter formulas with parentheses?
Use standard chemical notation: Ca(OH)2, Al2(SO4)3, (NH4)2SO4. The calculator handles nested parentheses up to 3 levels deep and correctly distributes multipliers.
How accurate are the results?
Results use IUPAC 2021 standard atomic weights and are computed to 3 decimal places. Mass percentages sum to 100% within rounding tolerance. The underlying atomic weight data is the same used in university-level chemistry courses worldwide.
Can I use this for ionic compounds?
Yes. Enter the formula unit as written: NaCl, CaCO3, Fe2O3, KMnO4. The calculator sums all atomic masses regardless of bonding type — percent composition depends only on atomic masses and counts.