What is an equilateral triangle?
An equilateral triangle has three equal sides and three equal angles of 60°.
The answer
An equilateral triangle has 3 sides, 3 vertices, no parallel sides and 3 lines of symmetry. The angles inside an equilateral triangle add up to 180°.
Equilateral triangle diagram and properties
| Property | Count |
|---|---|
| Sides | 3 |
| Vertices (corners) | 3 |
| Angles add up to | 180° |
| Each angle | 60° |
| Right angles | 0 |
| Pairs of parallel sides | 0 |
| Lines of symmetry | 3 |
Key facts about the equilateral triangle
- Every angle is 60°, because 180 ÷ 3 = 60.
- It is the regular triangle: all its sides and angles are equal.
- Six of them fit together round a point to make a regular hexagon.
Where you see it
A triangle warning road sign, a snooker ball rack and the sections of many climbing frames are equilateral.
The common mistake
Thinking the triangle must stand on a flat side. Turned any way round, three equal sides still make it equilateral.
An 11+ style question
An equilateral triangle has a perimeter of 27 cm. How long is each side, and what is each angle?
Answer: Each side is 9 cm and each angle is 60°
- The three sides are equal, so 27 ÷ 3 = 9 cm.
- The three angles are equal and add up to 180°, so 180 ÷ 3 = 60°.
Related shapes and skills
More shapes
Questions parents ask
How many sides and vertices does an equilateral triangle have?
An equilateral triangle has 3 straight sides and 3 vertices (corners). Its angles add up to 180°.
How many lines of symmetry does an equilateral triangle have?
An equilateral triangle has 3 lines of symmetry.
Is an equilateral triangle also isosceles?
By the wider definition, yes: it has at least two equal sides. Primary schools usually keep the names separate, so an isosceles triangle is taught as having exactly two equal sides.
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.