What is a circle?
A circle is a perfectly round flat shape, where every point on its edge is the same distance from the centre.
The answer
A circle has 1 curved edge, no straight sides, no vertices and infinitely many lines of symmetry.
Circle diagram and properties
| Property | Count |
|---|---|
| Straight sides | 0 |
| Curved edges | 1 |
| Vertices (corners) | 0 |
| Lines of symmetryAny straight line through the centre. | Infinitely many |
Most primary schools say a circle has 1 curved side (or edge) and no vertices, because it has no corners. It is not a polygon, because a polygon has straight sides.
Key facts about the circle
- The radius is the distance from the centre to the edge.
- The diameter goes right across the circle through the centre. It is always twice the radius.
- The circumference is the distance all the way round the edge: the perimeter of a circle.
- Any straight line through the centre is a line of symmetry, so a circle has infinitely many.
Where you see it
A clock face, a coin, the top of a mug and a bicycle wheel.
The common mistake
Mixing up the radius and the diameter. The radius goes from the centre to the edge; the diameter goes all the way across, so it is twice as long.
An 11+ style question
A circle has a radius of 7 cm. What is its diameter?
Answer: 14 cm
- The diameter is twice the radius.
- 2 × 7 = 14, so the diameter is 14 cm.
Related shapes and skills
More shapes
Questions parents ask
How many lines of symmetry does a circle have?
Infinitely many. Any straight line through the centre splits it into two matching halves.
How many sides does a circle have?
Most primary schools say 1 curved side (or edge), and no straight sides. It has no corners, so it has 0 vertices.
What is the difference between radius, diameter and circumference?
The radius goes from the centre to the edge. The diameter goes right across through the centre, twice the radius. The circumference is the distance all the way round.
When do children learn the parts of a circle?
Year 6 names the radius, diameter and circumference, and learns that the diameter is twice the radius.
Shape questions are quick marks in the 11+
Angles, symmetry, nets and 3D shapes come up in 11+ maths and non-verbal reasoning. The free diagnostic checks maths, English and both kinds of reasoning, and shows what to practise next. No card is required.