Percent Composition Calculator
Enter a chemical formula to see how much of its mass comes from each element.
Percent composition
H₂O
18.015 g/mol
| Element | Atoms | Atomic mass | Mass contribution | % by mass |
|---|---|---|---|---|
| H | 2 | 1.008 | 2.016 | 11.19% |
| O | 1 | 15.999 | 15.999 | 88.81% |
Total: 100.00%
Based on the atomic weights used by Chemistry Inspect. How atomic weights work
How it works
How percent composition is calculated
Count the atoms
Read the formula and determine how many atoms of each element it contains.
Find each element's mass
Multiply each element's atom count by its atomic mass.
Add the masses
The total gives the compound's molar mass.
Calculate each percentage
Divide each element's mass contribution by the total molar mass and multiply by 100.
% by mass = (element mass contribution ÷ total molar mass) × 100
What is percent composition?
Percent composition tells you what percentage of a compound's mass comes from each element.
For example, water is made from hydrogen and oxygen. Hydrogen makes up 11.19% of water's mass, while oxygen makes up 88.81%. The percentages describe the compound's theoretical composition from its chemical formula. They do not measure the purity of a real sample.
Concepts
Mass shares, clearly
Percent by mass is not atom percentage
The number of atoms and their share of the mass are different. H₂O has 2 hydrogen atoms and 1 oxygen atom, but oxygen contributes most of the mass because an oxygen atom is much heavier than a hydrogen atom.
Mass contribution
Each element contributes atom count × atomic mass. Those contributions add up to the compound's total molar mass.
Why percentages add to 100%
Every element in the formula accounts for part of the compound's total mass, so the percentages add to 100%, apart from display rounding.
Coefficients do not change composition
H₂O and 2H₂O have the same percentage composition because the coefficient multiplies every element equally.
Examples
Try an example
| Formula | Molar mass | Composition |
|---|---|---|
| 18.015 g/mol | H 11.19%, O 88.81% | |
| 58.440 g/mol | Na 39.34%, Cl 60.66% | |
| 44.009 g/mol | C 27.29%, O 72.71% | |
| 100.086 g/mol | Ca 40.04%, C 12.00%, O 47.96% | |
| 180.156 g/mol | C 40.00%, H 6.72%, O 53.28% | |
| 342.132 g/mol | Al 15.77%, S 28.11%, O 56.12% | |
| 249.677 g/mol | Cu 25.45%, S 12.84%, O 57.67%, H 4.04% |
FAQ
Frequently asked questions
What is percent composition?
Percent composition is the percentage by mass that each element contributes to a compound.
How do you calculate percent composition?
Find each element's mass contribution, divide it by the compound's total molar mass, and multiply by 100.
What is the difference between percent composition and atom count?
Atom count tells you how many atoms of each element are present. Percent composition tells you how much of the compound's total mass comes from each element.
Should percent composition add up to 100%?
Yes. The percentages should add up to 100%. Display rounding can sometimes cause a tiny difference.
How do I calculate the percent composition of H₂O?
Water has a molar mass of 18.015 g/mol. Hydrogen contributes 2.016 g/mol, giving 11.19%, while oxygen contributes 15.999 g/mol, giving 88.81%.
Can I use parentheses in a formula?
Yes. Formulas such as Ca(OH)₂ and Al₂(SO₄)₃ are supported.
Can I calculate percent composition for a hydrate?
Yes. Enter the hydrate normally, such as CuSO₄·5H₂O. The water molecules are included in the total mass.
Does a coefficient change percent composition?
No. H₂O and 2H₂O have the same percentage composition because the coefficient multiplies every element equally.
Why does my answer differ slightly from another calculator?
Small differences can come from the atomic-weight dataset or from rounding. Chemistry Inspect keeps full precision internally and rounds only the displayed result.
Is percent composition the same as purity?
No. Percent composition is the theoretical elemental makeup of a compound based on its formula. Purity describes how much of a particular substance is actually present in a sample.