Finding the Diameter from the Circumference
(KS3, Year 7)

The Lesson

The diameter of a circle is found from the circumference of a circle using the formula:

diameter equals circumference divided by pi In this formula, C is the circumference of the circle. The image below shows what we mean by finding the diameter from the circumference:

circumference and diameter on a circle

How to Find the Diameter from the Circumference

Finding the diameter from the circumference is easy.

Question

What is the diameter of the circle, with circumference 30 cm, below?

Step-by-Step:

1

Start with the formula:
Diameter = Cπ
Don't forget: / means ÷ and π is pi (≈ 3.14)

2

Substitute the circumference into the formula. In our example, C = 30.
Diameter = 30π Diameter = 30 ÷ π Diameter = 30 ÷ 3.14 Diameter = 9.6 cm

Answer:

The diameter of the circle is 9.6 cm.

Lesson Slides

The slider below shows another real example of how to find the diameter of a circle from the circumference. Open the slider in a new tab

Interactive Widget

Here is an interactive widget to help you learn about finding the diameter of a circle from its circumference.
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See Also

What is the diameter? What is a circle? What is the circumference? Finding the circumference of a circle Find the diameter from the circumference of a circle ( interactive widget)