Enter the value of any parameter of a circle (diameter, area or circumference) in the fields of the Circle Calculator and calculate the other two parameters.
Circle formulas
A circle is a plane figure bounded by one line, and such that all right lines drawn from a certain point within it to the bounding line, are equal. The bounding line is called its circumference and the point, its centre.
— Euclid. Elements Book I.
The Radius of a circle is the distance from the center of the circle to any point on the circle itself, which is half a diameter. The Diameter of a circle is a line segment whose endpoints lie on the circle and which passes through the center. The Circumference of a circle (or perimeter of a circle) is the distance around the circle.
Length of circumference of a circle.
The ratio of a circle’s circumference to its diameter is \(\pi\) (pi), an irrational constant approximately equal to 3.141592654. So the length of the circumference \(C\) is related to the radius \(R\) and diameter \(D\) by the following circle circumference formula:
$$C= 2\pi R= \pi D .$$
Area of a circle.
As is known from geometry, the area enclosed by a circle of radius \(R\) is defined by the folowwing circle area formula:
$$Area= \pi { R }^{ 2 }= \pi { D }^{ 2 }/4 .$$
Related calculators
Check out our other geometry calculators such as Triangle Calculator or Sphere Calculator.