Percent Composition Calculator
Enter a chemical formula to break it down into the percent by mass contributed by each element.
by mass (O)
180.16
g/molO
53.3%
C
40%
H
6.7%
- 1
Molar mass
180.156Sum of each element's atom count multiplied by its atomic weight (g/mol). - 2
O mass contribution
6 × 15.999 = 95.994 - 3
% of O (most abundant)
(95.994 ÷ 180.156) × 100 = 53.28
How does this calculator work?
Percent composition is the mass fraction of each element in a compound. For each element, multiply its atom count by its atomic weight, divide by the compound's total molar mass, and multiply by 100. Enter a formula like C6H12O6 and the tool returns every element's percent by mass, summing to 100%.
Formula
How this is calculated
Type a chemical formula such as C6H12O6 (glucose), H2O, or Ca(OH)2. The parser reads element symbols (one uppercase letter, optionally followed by lowercase letters), the subscript counts that follow them, and any parenthesized groups with their multipliers, expanding nested groups recursively.
For every element the calculator multiplies its atom count by its standard atomic weight (in grams per mole) to get that element's mass contribution. Summing those contributions gives the compound's molar mass. The percent composition of each element is then its mass contribution divided by the total molar mass, multiplied by 100. By definition these percentages add up to 100%.
Atomic weights use IUPAC standard values and are reported to several significant figures, so results are accurate to roughly two decimals. Hydrates can be entered with their water as a parenthesized group. Unknown symbols, lowercase-only tokens, or unbalanced parentheses return no result; charges and isotope labels are not parsed.
Frequently asked questions
Write them just like the formula, e.g. Ca(OH)2 or Al2(SO4)3. The subscript after a closing parenthesis multiplies every element inside that group.
Each element's percent is its share of the total molar mass, so the shares of all elements necessarily sum to 100% (small rounding differences aside).
Molar mass is the total grams per mole of the compound. Percent composition splits that total into the fraction contributed by each element, expressed as a percentage.
Also known as
TG we-Calculate Editorial Team. (2026). Percent Composition Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/percent-composition-calculator
TG we-Calculate Editorial Team. "Percent Composition Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/percent-composition-calculator.
TG we-Calculate Editorial Team, "Percent Composition Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/percent-composition-calculator
@misc{wecalculate_percent_composition_calculator, title = {Percent Composition Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/percent-composition-calculator}}, year = {2026}, note = {TG we-Calculate} }
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