Radius (r)
in
Diameter (d)
in
Circumference (c)
in
Area (a)
in²

Circle Calc: find c, d, a, r

This Circle calc finds c (circumference), d (diameter), a (area), and r (radius) of a circle.

This area of a circle calculator will help you determine the circumference, diameter, area, and radius of any circle. In the article below, you will find all equations you will ever need to find one of these values.

We use the following notation:

  • c stands for circumference,
  • d for diameter,
  • a for area, and
  • r for radius of the circle.

If you already know how to find these values, try the centrifugal force calculator for the application of these formulas in physics, or the circle skirt calculator for a more practical use.

Circle calc: find c

c is the circumference of the circle - that is, the total length of the line that you draw with a compass. You can calculate it in the following ways:

  1. If you know the radius or diameter of the circle:

c = 2πr = πd

  1. If r = n/a (radius unknown) and d = n/a (diameter unknown):

c = 2√(πa)

Circle calc: find d

d is the diameter of the circle - the length of a line that passes through the center point and joins two points on the circle. You can determine it with these equations:

  1. If you know the radius of the circle:

d = 2r

  1. If r = n/a (radius unknown) and a = n/a (area unknown):

d = c / π

  1. If r = n/a (radius unknown) and c = n/a (circumference unknown):

d = 2√(a / π)

Circle calc: find a

a stands for the area of the circle. It is the only value that is expressed in squared units (for example, cm²). You can calculate it as follows:

  1. If you know the radius or diameter of the circle:

a = πr² = π * (d / 2)²

  1. If r = n/a (radius unknown) and d = n/a (diameter unknown):

a = c² / 4π

Circle calc: find r

r, or the radius of the circle, is the length of a line that joins the center point with any point lying on the circle. It can be found with the following formulas:

  1. If you know the diameter of the circle:

r = d / 2

  1. If d = n/a (diameter unknown) and a = n/a (area unknown):

r = c / 2π

  1. If d = n/a (diameter unknown) and c = n/a (circumference unknown):

r = √(a / π)

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Circle Calc: find c, d, a, r