Percent Composition Calculator
See how much of a compound's mass each element accounts for. Enter the total mass and a row per element, and it returns the percent by mass for each.
Percent composition is each element's mass divided by the compound's total molar mass, times 100. Enter the total mass once, then a row per element. Water (18.015 g/mol) comes out as roughly 11% hydrogen and 89% oxygen.
Percentages add up to
100%
Total mass
18.015g/mol
If you've listed every element, the percentages should land close to 100%. A total that's noticeably off usually means a mass was mistyped or an element is missing from the list.
How it works
Percent composition breaks a compound down by mass: each element's mass divided by the compound's total molar mass, times 100. Water is about 11% hydrogen and 89% oxygen once you run the numbers.
Enter the compound's total mass once, then add a row for each element with the mass it contributes. Every row shows its own percentage live, and you can add or remove rows as your formula needs.
The tool also sums the percentages so you can sanity-check your work. List every element and they should add up close to 100%; if the total is off, a mass was probably mistyped or an element is missing.
Frequently asked questions
How do I find each element's mass in a compound?
Multiply the atomic mass of the element by how many of its atoms appear in the formula. Water has two hydrogens at about 1.008 each, so hydrogen contributes roughly 2.016 g/mol.
What should the percentages add up to?
If you've entered every element in the compound, they should total close to 100%. A noticeable gap means a mass is wrong or an element is missing from your list of rows.
Can I use this for a compound with several elements?
That's what the add-element button is for. Keep adding rows until every element is listed, put the compound's full molar mass in the total field, and each row reports its share by mass.