What is a decagon?
A decagon is a flat shape with ten straight sides and ten corners.
The answer
A regular decagon has 10 sides, 10 vertices, 5 pairs of parallel sides and 10 lines of symmetry. The angles inside a decagon add up to 1440°.
Decagon diagram and properties
| Property | Regular |
|---|---|
| Sides | 10 |
| Vertices (corners) | 10 |
| Angles add up to | 1440° |
| Each angle | 144° |
| Right angles | 0 |
| Pairs of parallel sidesIrregular decagons may have fewer. | 5 |
| Lines of symmetryAn irregular decagon may have fewer. | 10 |
Key facts about the decagon
- The angles inside any decagon add up to 1440°.
- In a regular decagon every angle is 144°.
- A regular decagon has 5 pairs of parallel sides.
- "Dec" means ten, as in decade (ten years) and decimal (counting in tens).
Where you see it
Some coins from other countries, clock faces and table tops are decagons. Most of the time it is an 11+ angle question.
The common mistake
Forgetting the angle rule and guessing. Split the decagon into triangles from one corner: there are always 2 fewer triangles than sides, so 8 triangles here.
An 11+ style question
A regular decagon has sides of 4.5 cm. What is its perimeter?
Answer: 45 cm
- A regular decagon has 10 equal sides.
- 4.5 × 10 = 45, so the perimeter is 45 cm.
Related shapes and skills
More shapes
Questions parents ask
How many sides and vertices does a decagon have?
A decagon has 10 straight sides and 10 vertices (corners). Its angles add up to 1440°.
How many lines of symmetry does a decagon have?
A regular decagon has 10 lines of symmetry. An irregular decagon may have fewer.
What do the angles in a decagon add up to?
1440°. A decagon splits into 8 triangles from one corner, and 8 × 180 = 1440.
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.