Circle Calculator
Find a circle’s area, circumference and diameter from any one measurement.
Area
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Circumference
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Diameter
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Radius
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Area (exact, ×π)
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Results update as you type.
About this calculator
A circle calculator works out every property of a circle from a single known value — the radius, diameter, circumference or area. It uses radius r, with area = π r², circumference = 2 π r, and diameter = 2 r, converting from whichever value you provide.
Frequently asked questions
What is the formula for the circumference of a circle?
Circumference = 2 × π × radius, or equivalently π × diameter. A radius of 5 gives a circumference of about 31.42.
How do you find the radius from the area?
Rearrange area = π r² to r = √(area ÷ π). This tool does that automatically when you choose to enter the area.
Results are estimates for general guidance only, not financial, medical or tax advice.