# What is Circumference? - Definition & Equation

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• 0:00 Circumference
• 0:32 Circumference Equation
• 0:47 Diameter Equation
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Lesson Transcript
Instructor: Yuanxin (Amy) Yang Alcocer

Amy has a master's degree in secondary education and has taught math at a public charter high school.

In this lesson, you'll learn how to define and find the circumference of a circle using two equations: one that involves the diameter and another that involves the radius.

## Circumference

Our earth is a sphere, and if we sliced it in half, we'd end up with a circle. Circumference is the distance around a circle. We can measure the circumference of the earth by measuring the distance we'd cover if we walked all the way around the world. To understand circumference, we also need to understand the meaning of diameter and radius.

The diameter is the distance across the center of a circle, shown in green in this image. The radius is the distance from the center to the edge of a circle, shown in red.

## Circumference Equation

There are two ways to write the equation for the circumference.

You can use either of these equations, depending on whether the problem provides you with the diameter or the radius.

## Diameter Equation

There are two ways you can apply the equation for the circumference of a circle using the diameter.

The first is by using the equation to find the circumference given the diameter. For example, if a problem gives you a diameter of 7, you'd plug in the value of the diameter for d and replace pi with 3.14.

The second way to use the equation is when you need to find the diameter given a circumference, such as 21.98 inches. To do this, divide both sides by pi.

There are also two ways you can apply the equation for the circumference of a circle using the radius.

The first is by using the equation to find the circumference given the radius of a circle. For example, let's say the problem gives you a radius of 6 inches. You'd plug the 6 into the r variable and replace pi with 3.14.

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