Use our simple circle calculator to get any of the radius, diameter, circumference and area of a circle by entering one value in the form below.
How to use:
- Type a value in any one field (radius, diameter, circumference, or area)
- The other three values will be calculated automatically as you type
- Clear a field to start with a different value
- Units are not displayed, remember that if the radius, the diameter or the circumference are in unit u, the area will be in unit u2
Definition of a circle
A circle is the set of points in a plane that are at a distance r of a point called the center. This distance r is called the radius of the circle.
Equation of the circle
In a Cartesian coordinate system, the circle with the centre at coordinates (a, b) and with radius r is the set of all points (x, y) such that:
r^2 = \left( x-a \right)^2 + \left( y-b \right)^2Circle formulas
Diameter and radius of a circle
The diameter d of a circle with respect to its radius r is d = 2r
Circumference of a circle
The circumference C of a circle can be calcuted from its radius r or its diameter d as C = 2πr = πd
Area of a circle
The area A of a circle can be calculated from its radius r or its diameter d as A = πr² = πd²/4