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# Finding the Radius from the Diameter

(KS3, Year 7)

## The Lesson

The radius of a circle is half of the diameter.## How to Find the Radius from the Diameter

Finding the radius from the diameter is easy.## Question

What is the radius of the circle, with diameter 10 cm, below?## Step-by-Step:

# 1

Divide the diameter, 10 cm, by 2.

10 ÷ 2 = 5 cm

## Answer:

The radius of the circle is 5 cm.## Interactive Widget

Here is an interactive widget to help you learn about finding the radius of a circle from its diameter.## Why Is Knowing the Radius in Terms of the Diameter Useful?

Most of the properties of a circle - such as its circumference and area - can be expressed in terms of the radius or diameter. Most students remember the area of a circle with a radius**r**as:

But what if you are told the diameter

**d**instead? Divide it by 2 to find the radius. If you substitute

**for**

^{d}⁄_{2}**r**, a formula for the area of a circle in terms of the diameter can be found:

Read more about how to find the area of a circle using the diameter

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