What are the parts of a circle?

What is a circle?
By the end of KS2 children are expected to be able to identify the parts of a circle (circumference, radius and diameter) and begin to use formulae to calculate a circle's perimeter and area. We explain what parents need to know about this aspect of primary-school geometry.
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What is a circle?

A circle is a 2D curved shape, every point of which is the same distance from a fixed point in the centre.

When do children learn about circles in primary school?

Children begin learning about 2D shapes in Year 1, where they learn to recognise and name circles, triangles, squares and rectangles. 

They may also discuss the properties of different shapes, for example the fact that circles have curved sides and triangles have straight sides.

In Year 2, children start to look at 3D shapes and will be asked to recognise 2D shapes on these. For example, they will be asked to say how many circular faces they can see on a cylinder (there are two):

At the end of KS2, in Year 6, children will be asked to illustrate and name parts of circles, including radius, diameter and circumference.

What is the circumference of a circle?

The circumference is the measurement all the way around the outside edge of a circle.

What is the radius of a circle?

The radius is the distance from the centre of the circle to the edge of the circle.

What is the diameter of a circle?

The diameter is the straight line going through the centre of a circle, connecting two points on the circumference.

Y6 children may be given various circles and asked to measure their circumference (using string), radius and diameter.

They will learn that the diameter of a circle is twice the radius and may be introduced to the formulae to help them calculate the circumference of a circle (2 x π x r) and the area of a circle (π x r²).