Proportion 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
a ÷ b = c ÷ d
Where this usually goes wrong
Cross-multiply: ad = bc. Keep the same kind of quantity on the same side or the ratio inverts.
Worked example
3 is to 4 as 9 is to 12.
Those numbers are already in the boxes above — press Work it out to see it, then change them for your own.
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