Circle Calculator
Enter one known circle measurement and solve radius, diameter, circumference and area together.
Radius
Ideal flat circle, nonnegative known measurement and consistent units. A zero value gives the degenerate zero-size case. Results are numerical approximations using π.
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How this calculator works
A single measurement determines an ideal circle. Radius is half the diameter; circumference is twice π times radius; area is π times radius squared. The inverse area calculation first divides by π and then takes the nonnegative square root. This makes the tool useful when you know an area or boundary length but need another dimension. Use one consistent length unit and its square for area, rather than mixing centimetres with square metres.
Formula
Diameter d = 2r; circumference C = 2πr; area A = πr². Inverse radius: d/2, C/(2π) or √(A/π).
Assumptions & limitations
Ideal flat circle, nonnegative known measurement and consistent units. A zero value gives the degenerate zero-size case. Results are numerical approximations using π.
Worked example
A radius of 3 units gives diameter 6, circumference 6π ≈ 18.849556 and area 9π ≈ 28.274334 square units. Entering that area instead recovers the same radius.
Our approach to calculations ↗Common questions
Can I start with area instead of radius?
Yes. Choose Area and enter square units. The radius is the square root of area divided by π.
How is this different from Area Calculator?
Area Calculator handles multiple shape types. Circle solves all four circle measurements in either direction from one known value.
Why must area units be squared?
Area describes two dimensions. A radius in centimetres produces area in square centimetres, not ordinary centimetres.