How this calculator works
A circle’s area grows with the square of its radius: doubling the radius quadruples the area. The calculation uses π to 15 decimal digits, converts your chosen length unit first, and lets you choose the area result unit. A radius of zero is valid and gives an area of zero.
Formula
Area = πr², with π ≈ 3.141592653589793.
Worked example
A radius of 3 gives an area of π × 9 ≈ 28.2743. A radius of 2.5 gives π × 6.25 ≈ 19.635.
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Frequently asked questions
Can I enter the diameter instead?
Halve the diameter first, since the formula needs the radius. The circumference calculator on this site also accepts a diameter directly.
Why is the area in square metres?
Inputs are converted to metres before calculating, so every result is reported consistently in square metres regardless of the unit you entered.
What happens if I enter a zero or negative radius?
Zero is accepted and gives an area of zero. A negative radius is rejected, so enter a length as a positive number.