Types of triangles
Triangles are named by their sides (equilateral, isosceles, scalene) and by their angles (acute, right, obtuse). Quiz for kids, parent help below.
🧒 Kids: what kind of triangle?
For sides, look for tick marks or equal numbers. For angles, look for a square corner or a wide angle!
▶ Quiz 🔷 More practice 📐 Area and perimeter
💡 Trick: 3 equal sides: equilateral. 2 equal: isosceles. 0 equal: scalene!
More on shapes and lines: Lines, rays and segments · Polygons · Types of triangles · Quadrilaterals · Parts of a circle · Faces, edges and vertices
This part is for grown-ups: the key idea, pictures to draw, what to say, common mistakes, questions with answers and free printables.
The trick: sides first, then angles
Ask “how many sides are equal?” first: 3, 2 or none. Then ask “is there a right angle, a wide angle, or neither?” The two answers give both names.
- Sides 5, 5, 5: equilateral
- Sides 7, 7, 4: isosceles
- Angles 90°, 40°, 50°: right
- Angles 120°, 30°, 30°: obtuse and isosceles
Patterns to spot
- The angles in any triangle add up to 180°.
- A triangle can have at most one right angle or one obtuse angle.
- Equal sides sit opposite equal angles.
- Every equilateral triangle has three 60° angles.
- The longest side is always opposite the biggest angle.
- Any two sides add up to more than the third side.
Key words
- Equilateral: all 3 sides the same length, and all 3 angles 60°.
- Isosceles: 2 sides the same length, and 2 angles the same size.
- Scalene: no sides the same length.
- Acute triangle: all 3 angles smaller than 90°.
- Right triangle: one angle exactly 90°, shown by a small square.
- Obtuse triangle: one angle bigger than 90°.
Why it matters: Triangles are the strongest shape, which is why bridges and roofs use them. Sorting them by sides and angles is the first step to finding missing angles and areas.
See it: pictures to draw with your child
Draw these on paper, or build them with straws, paper and boxes. Holding and turning real shapes makes the words stick.
What your child already knows
Your child already knows what a triangle is and what a square corner looks like. This topic sorts triangles using those two ideas: how many sides match, and what kind of corners they have.
In real life: Roof trusses, coat hangers, slices of pizza and the sails of boats are all triangles. Ask which kind each one is: are two sides the same?
Two names at once
Every triangle has a name by its sides and a name by its angles. A triangle with a right angle and two equal sides is a right isosceles triangle.
| Acute | Right | Obtuse | |
|---|---|---|---|
| Equilateral | Yes: always | Impossible | Impossible |
| Isosceles | Yes | Yes: 90°, 45°, 45° | Yes |
| Scalene | Yes | Yes: like a 3-4-5 triangle | Yes |
An equilateral triangle can only be acute, because all three angles are 60°. That is why two of the boxes say impossible.
Worked examples
Example 1: By sides
- A triangle has sides of 8 cm, 8 cm and 5 cm.
- Two sides are equal, so it is isosceles.
Example 2: By angles
- A triangle has angles of 30°, 60° and 90°.
- One angle is 90°, so it is a right triangle.
Example 3: Both names
- A triangle has angles of 100°, 40° and 40°.
- One angle is over 90°, so it is obtuse.
- Two angles match, so it is also isosceles.
Common mistakes
What to say to your child
A short script for your first session. Pause after each line and let your child answer.
- “How many sides are the same length?”
- “So is it equilateral, isosceles or scalene?”
- “Is there a square corner?” (Then it is a right triangle.)
- “Is any angle wider than a square corner?” (Then it is obtuse.)
- “If not, all the angles are sharp. What is it called?” (Acute.)
- “So what are its two names?”
Questions to try (answers below)
Ask these out loud, with pictures or real objects to hand. The last one is a stretch.
- Sides 4 cm, 4 cm, 4 cm
- Sides 6 cm, 6 cm, 3 cm
- Sides 3 cm, 4 cm, 5 cm
- Angles 90°, 60°, 30°
- Angles 50°, 60°, 70°
- Angles 110°, 35°, 35°
- Can a triangle have two right angles?
- What are the angles of an equilateral triangle?
- Stretch: a triangle has angles 45° and 45°. What is the third angle and its type?
Show the answers
- equilateral
- isosceles
- scalene (and right)
- right
- acute
- obtuse and isosceles
- No: that would already make 180°
- 60°, 60°, 60°
- 90°: a right isosceles triangle
Games to play at home
Straw triangles
Cut straws to different lengths and make triangles. Which ones are isosceles? Can you make an equilateral one?
Corner check
Use the corner of a page to test each angle of a drawn triangle: smaller, the same or bigger?
Triangle snap
Draw triangles on cards with their side or angle labels. Turn two over: same type? Snap!
A 5-day plan for types of triangles
Ten minutes a day is enough. Short, regular practice works far better than one long session.
- Day 1: Learn it with the pictures above: draw them on paper or build them with real objects, and talk them through.
- Day 2: Read the script together, then ask the first four questions.
- Day 3: Do the quiz at the top of the page and read the working for anything missed.
- Day 4: Play one of the games at home for ten minutes.
- Day 5: Ask the rest of the questions, then do a round of the matching practice skill.
By grade
Grade 4 (Common Core 4.G.A.2) sorts triangles by right angles and by their sides. Grade 5 (5.G.B.3, 5.G.B.4) sorts them into families and sees that every equilateral triangle is also isosceles.
CBSE families: Angles as Turns (class 5), A Tale of Three Intersecting Lines (class 7) and Triangles (class 10) build on these names.
More for each grade: Grade 4 math and the CBSE class 4 maths guide; Grade 5 math and the CBSE class 5 maths guide.
What comes next
Next, find a missing angle using the 180° rule in measuring angles, then the area of triangles. Compare with the quadrilateral family.
Worksheets and more practice
Print a free worksheet with an answer key, or keep practicing online with geometry practice.
Types of angles
Area of trianglesMore printables: classifying triangles worksheet · angles in a triangle worksheet · make a fresh test
More: geometry for kids · shapes for kids · practice · 2D shapes chart · 3D shapes chart
Last reviewed
Questions parents ask
What are the types of triangles?
By sides: equilateral, isosceles and scalene. By angles: acute, right and obtuse. Every triangle has one name of each kind.
What is an isosceles triangle?
A triangle with two sides the same length. The angles opposite those sides are also equal.
Can a right triangle be isosceles?
Yes. A triangle with angles 90°, 45° and 45° is a right isosceles triangle.
Can an equilateral triangle be obtuse?
No. All its angles are 60°, so it is always acute.
What grade learns types of triangles?
Grade 4 (4.G.A.2), with more sorting in grade 5 (5.G.B.4).