Maths
What are the parts of a circle?
Quick answer
A circle has a centre, a radius, a diameter and a circumference. The radius runs from the centre to the edge, and the diameter is twice the radius.
In one sentence
A circle is a round shape in which every point on the edge is the same distance from the centre, and each part has its own name.
Explained
Take a compass, a piece of string or a plate and trace around it. You have a circle. Every point on the edge is exactly the same distance from the middle. That distance is the key to the circle's parts.
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- Centre: the middle point.
- Radius: a straight line from the centre to the edge. The plural is radii.
- Diameter: a straight line across the circle through the centre. It is twice the radius: d = 2r.
- Circumference: the distance around the edge. It is the circle's perimeter.
- Chord: a straight line joining any two points on the edge. The diameter is the longest chord.
- Arc: a part of the circumference, like a bent piece of the edge.
- Sector: a slice, like a pizza slice, between two radii and an arc.
- Semicircle: half a circle, cut along a diameter.
- Tangent: a straight line that just touches the circle at one point.
The circumference is always a little more than three times the diameter. This special number is pi (π), about 3.14. So the circumference is C = π × d. A circle with a diameter of 10 cm has a circumference of about 31.4 cm. You can measure it with string and check.
To find how much space a circle covers, see the area of a circle. To compare circles with other shapes, read about 2D shapes.
Examples
| Radius | Diameter | Circumference (about) |
|---|---|---|
| 3 cm | 6 cm | 18.8 cm |
| 5 cm | 10 cm | 31.4 cm |
| 10 cm | 20 cm | 62.8 cm |
| 50 cm | 100 cm | 314 cm |
Common mistakes
- Mixing up radius and diameter. Always check which one a question gives. Radius is the short one, from the centre.
- Measuring the diameter off-centre. If a line misses the centre, it is a chord, not the diameter.
- Calling a circle an oval. Ovals have two different widths; a circle has just one.
Quick quiz
Parts of a circle
Check what you know with 4 quick questions.
Question 1 of 4
How is the diameter related to the radius?
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Related topics
Pi (π)
Pi (π) is the number you get when you divide any circle's circumference by its diameter. It is about 3.14, and its digits go on forever without repeating.
2D shapes
2D shapes are flat shapes with length and width but no thickness, like circles, triangles, squares and hexagons. You can draw them on paper.
Area of a circle
The area of a circle is pi times the radius squared: A = πr². A circle with radius 5 cm has an area of about 78.5 square centimetres.
Angles
An angle measures the amount of turn between two lines that meet at a point. Angles are measured in degrees (°); a right angle is 90° and a full turn is 360°.
Area
Area is the amount of flat space inside a shape, measured in square units like cm² or m². A rectangle 4 cm by 3 cm has an area of 4 × 3 = 12 cm².
Coordinates
Coordinates are a pair of numbers that give an exact position on a grid, like (3, 5). The first number is how far across (x) and the second is how far up (y).