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

## Note the 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

## Multiply Radius by Two

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 4 x 2 = 8. The diameter of the circle is 8 centimeters.

## Find Diameter From Circumference

## Note the Formula

The formula to find the circumference of a circle is C = πd. In other words, the diameter multiplied by pi constant equals the circumference.

## Remember Pi Constant

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.

## Divide Circumference by Pi Constant

If you know the circumference of a circle is 20 centimeters, work out 20 ÷ 3.14 = 6.37. The diameter of the circle is 6.37 centimeters.