# Free Math Lesson! How to Use Pi to Calculate the Circumference of a Circle

## Do You Need a Quick Lesson on How to Use Pi to Calculate the Circumference of a Circle?

The formula for the circumference of a circle is the diameter multiplied by pi, or D*pi. If you know the diameter, then you can use pi to find the circumference of a circle with little trouble. Most of the time in a math problem like this, you will be given the diameter, or at least a way to figure out the diameter. Then it's just one easy multiplication problem to get the answer you need.

## What Are Circumference and Diameter?

The circumference of a circle is the distance around the perimeter. In other words, it's the distance an ant would walk if it went around the whole circle once. The diameter is simply the distance across the middle of the circle at its widest point. It's the distance an ant would walk if it went in a straight line from one side of a circle across to the other.

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## What is Pi?

Pi is a number that basically never ends: 3.1415926535... and so on into eternity. It is used to find measurements in circles and other shapes with curved sides, like balls and tubes (i.e. spheres and cylinders). You use it in formulas, which in this case are equations you use to figure out measurements of shapes. Since pi is always the same number, it is considered the constant in an equation. You put measurements like diameter (D) or radius (r) into the formula. Those measurements can always be different from circle to circle, but pi stays the same.

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## No Matter Which Version of Pi You Use, It's Always the Same Idea

One thing about pi that can be confusing is this: When you use a formula like D*pi, it is generally okay to shorten pi to 3.14. Your school may have a different standard: 3.1415 is not uncommon. You may also use the "pi" button on your calculator. This will give you a more exact answer. But it's important to remember that no matter what "version" of Pi you use, the number and the idea is the same. The formula for finding the circumference of a circle is the diameter times pi.

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## An Example of Using Pi to Find the Circumference of a Circle

Let's say you have a circle with a diameter of 8 inches. To use pi to find the circumference of the circle, you take 8 and multiply it by pi -- it's that easy! Let's use the 3.14 "version" of pi: Multiply 8 times 3.14.

8 * 3.14 = 25.12. Your circle has a circumference of 25.12 inches.

(Be sure you include the unit of measurement! In the problem above it was inches, but it may be anything from millimeters to miles!)

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