Maths – Circle

The circle is a 2D (flat) shape that can be made by tracing a curve that is always the same distance from a point that we call the centre.

Maths – Circle Facts


The centre of a circle is the point in the very middle.

The radius of a circle is a line from the centre of the circle to a point on the side. The radius is the same length all the way around the circle. Mathematicians use the letter r for the length of a circle’s radius.

The diameter of a circle is a straight line that goes from one side to the other, passing right through the centre of the circle. Diameter means ‘all the way across’. Mathematicians use the letter d for the length of this line.

The diameter of a circle is equal to twice its radius. (d = 2 x r or d = 2r)

The circumference of a circle is the distance all the way around it. Circumference means ‘all the way around’. Mathematicians use the letter c for the length of this line.

An arc is part of the circumference of a circle. The full arc of a circle measures 360 degrees.

A chord is a straight line joining two points on a circle. The diameter is an example of a chord.

A semicircle is a shape that forms half a circle.

Circles have a high level of symmetry.

Good to know!

The circle shape is a favourite of humans and can be seen in many designs.

The invention of the wheel (a circle shape) was one of the most important in human history.

Learning resources

Example questions

  • Tell me the definition of a circle.
  • What does ‘diameter’ mean?
  • What is an arc?
  • What is a chord?
  • Tell me a fun fact about circles.
  • What is the name of a triangle whose sides are all of equal length?
  • What makes a triangle a scalene triangle?
  • Tell me about the angles in an acute-angled triangle.
  • What is the process of measuring using triangles called?
  • Other than the circle and the triangle, name two geometric plane shapes.

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