# How to Determine the Diameter of a Circle

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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.

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

3. ## Multiply Radius by Two

4. 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 × 2 = 8

The diameter of the circle is 8 centimeters.

1. ## Note the Formula

2. The formula to find the circumference of a circle is

C = πd

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

3. ## Remember Pi is a Constant

4. 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.

5. ## Divide Circumference by Pi Constant

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

\frac{20}{3.14} = 6.37

The diameter of the circle is 6.37 centimeters.