Percent Composition Calculator
Work out each element's percentage by mass in a compound.
What is Percent Composition?
Percent composition says what fraction of a compound's mass comes from each element — glucose is 40.0% carbon by mass, 6.7% hydrogen and 53.3% oxygen. It is the first step of formula determination going forwards, and the way to check whether a measured elemental analysis is plausible going backwards.
How it works
Each element's contribution (atomic mass × count) is divided by the molar mass and multiplied by 100. The percentages always total 100, which is the check worth making: if they do not, a subscript has been misread somewhere. The table appears twice because the two orderings answer different questions — 'how much oxygen' against 'in the order I typed it'.
- Enter what you know. Every field recalculates as you type — there is no submit button. Leave the quantity you want blank and the page solves for it.
- Read the answer and the working. The result appears above the fold; underneath it is the formula with your numbers substituted, so the arithmetic can be checked rather than trusted.
- Check the units and the size. Units are shown on every value and selectable per field, so a result in kJ, kPa or km/h is converted rather than assumed. Sanity-check the magnitude against a known landmark such as 1 g/cm³ for water.
Examples
A textbook problem with a known answer
Every tool here is checked against standard example problems — the molar mass of C6H12O6 is 180.16 g/mol, and free fall from 20 m takes about 2.02 s at g = 9.8 m/s².
A result you want to reason backwards from
Leave the quantity you already know blank instead: given the final answer, most tools recover the starting value — the mass needed for a target concentration, or the height a fall must have had.
Common mistakes
Forgetting that a formula is only as good as its subscripts
H2O and H2O2 are different compounds. The formula field shows its parse — element by element — so a mis-typed subscript is visible before it reaches the arithmetic.
Mixing units in the middle of a calculation
The most common source of a wrong answer is a value in grams next to a formula expecting kilograms, or Celsius next to a gas constant expecting kelvin. This category converts on the way in and labels what it used.
Frequently asked questions
What is the percent composition of water?
88.81% oxygen and 11.19% hydrogen by mass. The water molecule is two-thirds hydrogen by atom count but only about one-ninth by mass, which is exactly the distinction this tool makes.
Do the percentages have to add up to 100?
Yes, within rounding. The total is displayed so a mis-typed subscript — which usually makes them add to something else — is obvious.