Circle area 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 = πr²
Where this usually goes wrong
Radius, not diameter. If the question gives you the width across, halve it first.
Worked example
A radius of 5 gives an area of 78.54.
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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