Percent composition calculator
Fill in what you know and leave the unknown box empty — it works out whichever one is missing. Nothing is uploaded; the arithmetic happens in your browser.
The formula
% = (part ÷ whole) × 100
Where this usually goes wrong
Use the mass of the element across the whole formula — in H₂O the hydrogen part is 2 × 1.008, not 1.008.
Worked example
Water is 88.8% oxygen by mass.
Those numbers are already in the boxes above — press Work it out to see it, then change them for your own.
Related chemistry calculators
- Molarity calculator
- Dilution calculator
- Percent yield calculator
- Density calculator
- Ideal gas law calculator
- pH and pOH calculator
Want this everywhere, not just on this page?
StudyFrame is a Chrome extension. Snip any question on any site and get the answer in place — plus summaries, flashcards and practice tests.
Free to install. 3 uses a day at no cost, more if you study on a streak.