Radius and diameter
If the radius is 4 cm, the diameter is 8 cm.
The diameter is always twice the radius.
Year 6
Illustrate and name parts of circles, including radius, diameter and circumference, and know that the diameter is twice the radius
Name the main parts of a circle and use the relationship between radius and diameter.
Lesson overview
Geometry - properties of shape
The radius goes from the centre to the edge of a circle. The diameter goes all the way across through the centre, so it is twice the radius. The circumference is the distance right around the edge.
Circumference
The circumference is the distance around the circle.
centre to edge
edge through centre to edge
distance around
These names refer to different parts of the same circle.
Worked example
A circle has radius 6 cm. What is its diameter?
| Radius | Diameter |
|---|---|
| 6 cm | 12 cm |
- The diameter is twice the radius.
- Double 6 cm.
- 6 x 2 = 12.
- So the diameter is 12 cm.
Try it
Decide which part of the circle is being described before you answer.
Question 1
If the radius is 4 cm, the diameter is:
Question 2
The distance around a circle is called the:
Question 3
The line from the centre to the edge is the:
Question 4
The diameter is:
Question 5
If the diameter is 10 cm, the radius is:
Common mistakes
Watch for these when working through the lesson.
- Mixing up radius and diameter.
- Forgetting that the diameter goes through the centre.
- Calling the edge of the circle the diameter instead of the circumference.
Related topics
These ideas fit closely with this lesson.
- Compare and classify geometric shapes.
- Recognise angles where they meet at a point or on a straight line.