Circle Calculator
Calculate radius, diameter, area, and circumference from any one known value.
About Circle Calculations
A circle is defined by a single measurement — its radius (r), the distance from the center to any point on the edge. All other circle properties are derived from this fundamental value. The diameter is simply twice the radius (d = 2r), making it the longest chord in the circle passing through the center.
The circumference (C = 2πr) is the perimeter of the circle — the total length of its boundary. The area (A = πr²) measures the space enclosed within the circle. Both formulas involve π (pi), the mathematical constant approximately equal to 3.14159265, representing the ratio of a circle's circumference to its diameter.
This calculator allows you to start from any known measurement and derive all others. For instance, if you know the area of a circular field, you can find its diameter or circumference instantly. This is especially useful in engineering, construction, and physics where you may know one property but need another.
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For a related calculation, use Pythagorean Theorem Calculator to given any two sides of a right triangle, solve for the third using the Pythagorean theorem c²=a²+b². Alternatively, use Area Calculator to calculate the area of circles, triangles, rectangles, trapezoids, ellipses, parallelograms, and sectors.
For the underlying method, read How to Solve Any Triangle: Law of Sines vs. Law of Cosines.