Math
Circle Calculator
Calculate radius, diameter, circumference, and area of a circle from any one known value.
Circle Calculator
Circle properties
- Radius: 5
- Diameter: 10
- Circumference: 31.415927
- Area: 78.539816
Diameter formula
d = 2r
Circumference formula
C = 2pi r
Area formula
A = pi r^2
Frequently Asked Questions about the Circle Calculator
What formulas does this circle calculator use?
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.
What value of pi does the calculator use?
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.
What is the difference between radius and diameter?
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.
How do I find the radius from the circumference or area?
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.
What is the difference between a sector and a segment?
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.