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 standard relations: diameter d = 2r, circumference C = 2 pi r, and area A = pi r squared. Given any one of radius, diameter, circumference, or area, the calculator derives the others.
What value of pi does the calculator use?
It uses JavaScript's built-in Math.PI, which is accurate to roughly 15 decimal places, far more precise than the common 3.14 or 3.14159 approximations.
What is the difference between radius and diameter?
The radius is the distance from the center of the circle to any point on the edge. The diameter is the full distance across the circle through the center, so it is always exactly twice the radius.
How do I find the radius from the circumference or area?
From circumference: r = C / (2 pi). From area: r = sqrt(A / pi). The calculator handles both automatically when you pick the corresponding known field.
What is the difference between a sector and a slice (segment)?
A sector is the pie-shaped region bounded by two radii and the arc between them. A segment (sometimes called a slice) is the region bounded by a chord and the arc it cuts off. This calculator handles the full circle only.