Radius Calculator: Compute Dimensions of a Circle

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Below is a radius calculator, which will compute a circle's diameter, circumference, and area if you know the radius.

Do you know a different dimension? Instead try one of the other circle dimension calculators:

Radius Calculator

What is a Circle's Radius?

The radius of a circle is the distance from a circle's origin or center to its edge.

Conveniently, it is half as long as the diameter of a circle. A diameter is just two radiuses drawn in opposing directions from the circle's origin.

Dimensions of a Circle

For a circle, three lengths most commonly are applied:

  • The radius – defined above
  • The diameter – the distance from edge to edge of a circle passing through its origin or center. Twice the length of a circle's radius
  • The circumference – the length of the outside boundaries of the circle

If you know the radius, it is straightforward to compute the other two.

To find the radius from the diameter, you only have to divide by two:


If you know the circumference it is a bit harder, but not too bad:

Dimensions of a circle: O - origin, R - radius, D - diameter, C - circumference (Wikimedia)

Area, on the other hand, is all the space contained inside the circle. It's also straightforward to find the area if you know the radius:

a = \pi r^2

Using the Radius Calculator

You can enter the radius and then compute diameter and circumference in mils, inches, feet, yards, miles, millimeters, centimeters, meters and kilometers.

Area has different units, but you can use: square mils, square inches, square feet, square yards, square miles, acres, hectares, square millimeters, square centimeters, square meters, and square kilometers.

To run the computations, hit the 'Calculate Circle Dimensions' button when you have entered the known radius.

Have fun? Visit our other calculators and tools.



PK started DQYDJ in 2009 to research and discuss finance and investing and help answer financial questions. He's expanded DQYDJ to build visualizations, calculators, and interactive tools.

PK lives in New Hampshire with his wife, kids, and dog.

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