Online Tool Store
Percent Composition Calculator
Compute the mass percent of each element in a compound from its chemical formula and atomic masses.
This works right here on your device — nothing gets sent out.
Element symbols are case-sensitive. Parentheses, brackets and hydrate dots (CuSO4·5H2O) are supported.
Molar mass
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Step-by-step working
Example shown — glucose, C6H12O6. Atomic masses are IUPAC conventional standard values; results can differ slightly from tables that use other rounding. For radioactive elements the mass number of the longest-lived isotope is used.
How to calculate percent composition
- Choose your starting point: a chemical formula such as C6H12O6 or CuSO4·5H2O, or the measured mass of each element from a lab analysis.
- Enter the formula (and an optional sample mass), or add each element symbol with its mass in grams, and pick how many decimal places you need.
- Read the percent of each element in the table and chart, then open the step-by-step working to see every division, or copy the results.
The formula
Percent composition is the share of a compound's mass that comes from each element:
% of element = (atoms of the element × atomic mass) ÷ molar mass × 100 % of element (from lab data) = mass of the element ÷ total mass × 100
Example: glucose, C6H12O6, has a molar mass of 6 × 12.011 + 12 × 1.008 + 6 × 15.999 = 180.156 g/mol, so carbon is 72.066 ÷ 180.156 × 100 = 40.00%, hydrogen 6.71% and oxygen 53.28%. The percentages always add up to 100%. Atomic masses are the IUPAC conventional standard values, so figures can differ in the last digit from tables that round differently.
FAQ
What is percent composition?
Percent composition (also called mass percent) is the percentage of a compound's total mass that comes from each element in it. For water, H2O, that is 11.19% hydrogen and 88.81% oxygen.
How do you calculate percent composition from a formula?
Multiply each element's atomic mass by the number of its atoms in the formula, add those masses to get the molar mass, then divide each element's mass by the molar mass and multiply by 100. The working is shown step by step under the results.
How do you find percent composition from experimental data?
Choose "Measured masses of each element", enter the grams of each element found in your sample, and the tool divides each mass by the total. For example 4.80 g C, 0.806 g H and 6.40 g O gives about 39.98% C, 6.71% H and 53.31% O.
Do the percentages have to add up to 100%?
Yes, the exact values always do. Displayed values are rounded, so they can occasionally add to 99.99% or 100.01%; increase the decimal places to see the difference shrink.
How do I find the percent of water in a hydrate?
Enter the hydrate with a dot, such as CuSO4·5H2O. The tool detects the H2O part and shows the water of hydration as a percent of the whole mass, which is 36.08% for copper(II) sulfate pentahydrate.
How many grams of each element are in my sample?
Type your sample mass in grams and the table adds a column with the mass of every element in that sample, calculated as its percent times the sample mass.
Does the calculator send my formula anywhere?
No. Everything is calculated in your browser and nothing you enter is sent to a server.
How we compare
| Feature | Online Tool Store | Omni Calculator | Pearson |
|---|---|---|---|
| Type a full formula, including groups and hydrates | Yes | Element counts entered one by one, up to 5 elements | Yes, nested groups and hydrates |
| Full element breakdown with a chart | Yes, table and bars | Percent per element and molecular weight | Yes, table and chart |
| Step-by-step working | Yes | Worked example on the page | Yes |
| Adjustable decimal places | Yes, 1 to 4 | Not listed on the page | Yes |
| Grams of each element in a sample of your mass | Yes | Not listed on the page | Not listed on the page |
| Percent composition from measured lab masses | Yes | Guidance on reverse calculations | Not listed on the page |
Omni Calculator and Pearson are both good options, and Pearson also links to practice problems, which this tool leaves out. Choose Online Tool Store when you want a full formula or lab-data calculation, the working shown, and a sample-mass breakdown in one plain browser tool.