How to find the radius of a cone

A cone is a tapering three-dimensional geometric shape with a flat, circular face at one end and a point called the apex at the other end. An object which has a shape like a cone is often referred to as conical. Examples of some commonly found conical shapes are an ice-cream cone, a conical party hat and a traffic cone. Ellipses, which are geometrically related to cones, can be obtained by cutting up cones, according to mathematician J. W. Downs.

Let x be equal to the radius of the cone, as x is the thing you are attempting to discover. Write down “2 x =”.

Lay your ruler across the widest part of the circular face of the cone, which is known as the diameter. Write down the measurement after “2 x =”, for example: “2 x = 10 cm”.

Find x by dividing the diameter measurement by 2. For example, if the diameter is 10 cm, x = 5 cm. Use a calculator for this calculation if necessary. Note that the radius is half of the diameter, as it is a measurement from the edge of the circle to the centre point.

Write down “The radius of the cone is” and then write down the figure you arrived at in step 3.


If you know only two variables of the cone, use a Cone Calculator to calculate other properties.


The point of a cone is sharp and appropriate care should be taken when handling it.

Things You'll Need

  • Ruler
  • Pencil
  • Paper
  • Calculator (if necessary)
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About the Author

Frank Luger had his first educational resources published in the early 1990s. He worked on a major reading system for Cambridge University Press, became an information-technology adviser and authored interactive whiteboard resources for "The Guardian." Luger studied English literature and holds a Bachelor of Education honors degree from Leeds University.