A circle is the set of all points in a plane that are a fixed distance from a fixed point. The fixed distance is called the radius, and the fixed point is called the center of the circle. The diameter of a circle is any line segment that passes through the center of the circle and has its endpoints on the circle. You can easily find the diameter of a circle if you know its radius or circumference.

### Find Diameter From Radius

The radius is the fixed distance from the center of a circle to any point on its circumference. Record the radius of the circle

The radius of a circle always equals half of its diameter, so to find the diameter, multiply the radius by two. For example, if you know the radius of a circle is 4 centimeters, work out

The diameter of the circle is 8 centimeters.

### Find Diameter From Circumference

The formula to find the circumference of a circle is

In other words, the diameter multiplied by pi equals the circumference.

Pi, a never-ending string of numbers, is the ratio of the circumference of a circle to its diameter. This ratio stays the same no matter how big or small it is. For ease of use, pi is commonly shortened to 3.14.

If you know the circumference of a circle is 20 centimeters, work out

The diameter of the circle is 6.37 centimeters.

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Claire is a writer and editor with 18 years' experience. She writes about science and health for a range of digital publications, including Reader's Digest, HealthCentral, Vice and Zocdoc.