Calculate percent composition

Break a chemical formula into each element’s percentage by mass.

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ToolPercent Composition Calculator
Input
Output
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Preview

How it works

The parser expands counts, each contribution is count × atomic weight, and contribution divided by total formula mass gives mass percentage. Rounded display values therefore sum approximately to 100%.

  • Standard atomic weights are used.
  • Percentages are rounded only for display.

Worked example

Water
H2O split into hydrogen and oxygen by mass
Input
											H2O
Output
												Molar mass: 18.015
Breakdown: O: 88.81%, H: 11.19%

When to use this

Students break water into element percentages, materials reports compare composition, and assay checks compare formula with measured mass.

Edge cases

  • Parenthesized groups contribute every repeated atom.
  • Rounded visible percentages can sum to 99.99% or 100.01%.
  • Isotope-enriched samples can differ from standard-weight results.

References