Percent Composition Calculator
Calculate percent composition instantly in your browser — the published formula, worked locally with no data sent anywhere.
IUPAC atomic weights last updated · reference source
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Dated static reference; no live data is fetched. Verify current source values and assumptions before relying on results.
What this tool does
Calculate percent composition instantly in your browser — the published formula, worked locally with no data sent anywhere. Percentages are by mass and sum to 100% (within rounding). Percent composition is how an empirical formula is worked out from lab combustion analysis — the reverse of this calculation.
How to use the Percent Composition Calculator
- Enter or select chemical formula.
- Read the calculated result; change any measurement to compare alternatives.
Formula
percent composition: for each element, (atoms × atomic weight) ÷ total molar mass × 100, with the formula parsed and IUPAC atomic weights applied.
- formula
- Chemical formula
Percentages are by mass and sum to 100% (within rounding). Percent composition is how an empirical formula is worked out from lab combustion analysis — the reverse of this calculation.
Worked example
For percent composition calculator, the following measurements illustrate the exact method: Chemical formula: H2O.
Inputs
- Chemical formulaH2O
Result
- Percent composition by massH 11.19%, O 88.81%
- Molar mass (g/mol)18.02
Results explained
- Percent composition by mass
- Percent composition by mass from the formula above. Percentages are by mass and sum to 100% (within rounding). Percent composition is how an empirical formula is worked out from lab combustion analysis — the reverse of this calculation.
- Molar mass (g/mol)
- Molar mass (g/mol) from the formula above. Percentages are by mass and sum to 100% (within rounding). Percent composition is how an empirical formula is worked out from lab combustion analysis — the reverse of this calculation.
Frequently asked questions
percent composition: for each element, (atoms × atomic weight) ÷ total molar mass × 100, with the formula parsed and IUPAC atomic weights applied.
Percentages are by mass and sum to 100% (within rounding). Percent composition is how an empirical formula is worked out from lab combustion analysis — the reverse of this calculation.
Every field is labelled with its unit — enter values in exactly the labelled unit and read the result in the labelled output unit. Fields with a unit symbol also support the site-wide imperial/metric switch, which converts before the formula runs.
It uses double-precision arithmetic and the published constants and formulas named on this page. The last displayed digit may be rounded, and real conditions can differ from the idealized model described in the assumptions.
Yes. Every calculation runs entirely in your browser; nothing you enter is uploaded, stored or shared.