Circle Area Calculator
Calculate the area of a circle from its radius using Area = pi x r^2. Fast and accurate with support for any unit. Free online tool.
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How to use this tool
- Enter radius in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your area and the full breakdown beneath it.
Formula
Area = pi x r^2
How it works
Square the radius and multiply by pi (approximately 3.14159). The area is measured in square units.
Worked example
Circle with radius 5 m
- Area = pi x 5^2
- Area = pi x 25
- Area = 78.5398 m^2
78.5398 m^2
Common mistakes to avoid
- Entering the diameter instead of the radius; if you measure across the full circle, divide by 2 before entering.
- Using pi as 3.14 for precision-critical work; the calculator uses a full-precision value of pi.
- Squaring the area result instead of the radius, computing (pi x r)^2 instead of pi x r^2.
Key terms
- What is pi?
- Pi (approximately 3.14159) is the ratio of a circle's circumference to its diameter.
- Radius vs diameter?
- The radius is the distance from the center to the edge; the diameter is twice the radius.
Frequently asked questions
- I have the circumference. Can I find the area?
- Yes. Derive the radius first: r = C / (2 x pi). Then Area = pi x r^2. Or equivalently, Area = C^2 / (4 x pi).
- Does the formula change if I use diameter instead of radius?
- Yes. In terms of diameter d: Area = pi x (d/2)^2 = pi x d^2 / 4.
- What units does the area come out in?
- The area is in the square of whatever unit you used for the radius. Radius in cm gives area in cm^2.