Circle Calculator

It uses the three standard circle relations: diameter = 2 x radius, circumference = 2 x pi x radius, and area = pi x radius^2. Enter any one of radius, diameter, circumference, or area, and the other three are derived from it automatically.

It uses JavaScript's built-in Math.PI, which carries about 15 significant digits of precision, far more than the common 3.14 or 3.14159 shorthand. Results are rounded to 6 decimal places for display.

The radius is the distance from the center of a circle to any point on its edge. The diameter is the straight-line distance across the circle through the center, exactly twice the radius. For a circle with a 5-inch radius, the diameter is 10 inches.

Select the matching known field from the dropdown, enter your value, and the calculator solves for radius automatically. It uses r = C / (2 x pi) when you know the circumference, and r = sqrt(A / pi) when you know the area.

A sector is the pie-slice region bounded by two radii and the arc between them, like a pizza slice. A segment is bounded by a chord (a straight line between two points on the edge) and the arc it cuts off. This calculator covers the full circle only; it does not compute sectors or segments separately.