Parts of a circle

​​In a nutshell

A circle is a 2D plane shape where all the points are an equal distance from the centre.

Parts of a circle

​Circles can be described by diameter, radius and circumference.

​​​​Definitions

• ​Diameter - The distance, through the centre, across the circle (twice the radius).
  
• Radius - The distance from the edge to the centre of the circle (half the diameter).
• Circumference - The distance around the edge of the circle.

Insert image showing diameter, radius and circumference of a circle.

The diameter of a circle is double the length of the radius.  It can be written as:

​$$d = 2r$$

​$$r=\frac{1}{2}d$$​​

Where $$d$$ is the diameter of the circle, and $$r$$ is the radius of the circle.​

Example

The radius of a circle is $$4.8cm$$​. What is the diameter of the circle?

First, write out the known quantity:

$$r = 4.8 \space cm$$

Then, write the formula for the diameter of a circle:

$$d = 2r$$​​

Next, substitute the known quantity into the formula:

$$d = 2 \times 4.8$$

Don't forget to include the units:​

$$9.6 \space cm$$

The diameter of the circle is $$\underline{9.6cm}$$​