# How To Determine The Diameter Of A Circle

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

### 1. 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

### 2. 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

\(4 × 2 = 8\)

The diameter of the circle is 8 centimeters.

## Find Diameter From Circumference

### 1. Note the Formula

The formula to find the circumference of a circle is

\(C = πd\)

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

### 2. Remember Pi is a 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.

### 3. Divide Circumference by Pi Constant

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.

### Cite This Article

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Gillespie, Claire. "How To Determine The Diameter Of A Circle" *sciencing.com*, https://www.sciencing.com/determine-diameter-circle-5743525/. 16 November 2020.

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Gillespie, Claire. (2020, November 16). How To Determine The Diameter Of A Circle. *sciencing.com*. Retrieved from https://www.sciencing.com/determine-diameter-circle-5743525/

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Gillespie, Claire. How To Determine The Diameter Of A Circle last modified March 24, 2022. https://www.sciencing.com/determine-diameter-circle-5743525/