Percent Composition Calculator
Use this percent composition calculator to parse a chemical formula, calculate its molar mass, and show how much each element contributes by mass.
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Run the calculator.
What This Percent Composition Calculator Helps You Do
This calculator turns a molecular formula into atom counts, molar mass, and element-by-element mass percentages in one pass.
That makes it useful for empirical-formula work, stoichiometry checks, and fast validation when a percentage answer looks suspicious.
How to Calculate Percent Composition Calculator
- Enter the formula: Type a valid chemical formula such as H2SO4, Ca(OH)2, or CuSO4·5H2O.
- Parse the atom counts: The calculator expands groups and hydrate notation, then totals the atoms of each element.
- Convert to percentages: Each element's mass contribution is divided by the total molar mass to produce the percent composition.
Percent Composition Calculator Formula
| Variable | Meaning | Unit |
|---|---|---|
| % element | Mass percent of an element in the compound | % |
| m_element | Element mass contribution in one mole | g/mol |
| M | Compound molar mass | g/mol |
Use the worked examples below to check how the formula behaves with real values. If the result looks unexpected, verify the unit assumptions and the meaning of each variable before interpreting the answer.
Worked Examples
- Formula: H2O
Result: Hydrogen = 11.19%, oxygen = 88.81%
Most of water's mass comes from oxygen even though it has two hydrogen atoms.
- Formula: Ca(OH)2
Result: Ca = 54.09%, O = 43.19%, H = 2.72%
Parentheses matter because the OH group appears twice.
How to Interpret Your Results
| Range | Meaning | Action |
|---|---|---|
| Largest percentage | Dominant mass contributor | Use it to sanity-check whether the formula entry looks right. |
| Small percentage | Minor mass contributor | Small percentages are common for hydrogen or low atom counts. |
| All percentages sum to 100% | Balanced output | If they do not, the formula was not parsed correctly. |
Frequently Asked Questions
References
Last reviewed: March 2026