Find percent composition by mass
Percent composition describes how the molar mass of a pure compound is divided among its elements. Enter a chemical formula and the calculator expands every atom count, finds each element subtotal, divides that subtotal by the total molar mass, and multiplies by 100. The answer therefore reports mass percentage, not the percentage of atoms and not the concentration of a solution.
Sulfuric acid, H2SO4, illustrates the method. Hydrogen contributes the mass of two hydrogen atoms, sulfur contributes one sulfur atomic weight, and oxygen contributes four oxygen atomic weights. Each contribution is divided by the same compound molar mass. The result shows all percentages together and includes their rounded total so display rounding can be distinguished from a missing element.
The percent composition equation
For an element E, mass percent equals the mass contribution of E in one mole of compound divided by the compound molar mass, multiplied by 100. The element contribution is its expanded subscript multiplied by its conventional atomic weight. Because every element subtotal is included in the denominator, the unrounded shares sum mathematically to exactly 100 percent.
Displayed percentages are rounded independently, so their printed sum may be a few final decimal places above or below 100 percent. The result explicitly prints the displayed total rather than silently forcing one element to absorb the rounding difference. A material difference from 100 percent would indicate an omitted contribution or an invalid calculation and should not be explained away as rounding.
Use the exact formula for the material
Parentheses and hydrate coefficients can change composition substantially. Anhydrous CuSO4 and CuSO4·5H2O do not have the same copper percentage because the hydrate includes additional hydrogen and oxygen mass. Likewise, CaCl2 and CaCl2·2H2O answer different questions. Copy the formula from the assignment, label, or verified source and retain all stated waters of hydration or solvation.
A chemical formula is case-sensitive and must use known element symbols. The parser accepts nested groups, brackets, Unicode subscripts, and hydrate dots within bounded limits. It rejects malformed or ambiguous formulas rather than guessing. If the sample is a mixture, solution, impure reagent, polymer distribution, or material with variable stoichiometry, one fixed formula percentage may not describe the measured sample.
Check a percent composition result
First confirm that the expanded element counts match the formula. Then check that each percentage lies from zero to 100 and that the largest mass contribution usually produces the largest mass share. For water, oxygen must dominate by mass even though hydrogen atoms are more numerous. That contrast is a helpful reminder that atom count and mass contribution are different ideas.
You can reverse-check any row by multiplying the reported percentage as a decimal by the total molar mass. The result should reproduce the element subtotal within display rounding. Percent composition can support empirical-formula work, purity comparisons, and stoichiometric planning, but only when the formula and atomic-weight basis match the problem being solved.
Interpretation limits and private use
This calculation uses conventional atomic weights and assumes a pure substance with the entered formula. It does not measure a specimen, determine isotope abundance, calculate solution mass percent, infer an unknown formula, or certify a product composition. Laboratory composition requires an appropriate analytical method, uncertainty treatment, and sample context.
The values are calculated in the current browser tab. NexaCurrent does not upload the quantities, formulas, chemical names, element choices, or other information entered here, and no account is needed. Copying is a deliberate action after the answer and working are visible. Refreshing or leaving the page clears the current calculation, so keep a copied result if it is needed for later study.