Classifying angles

When you look at an angle you can name it by how big it is, even without measuring exactly. Year 4 uses four main names.

The four families

How to spot a right angle

A right angle is the special one. Tip: any square corner is a right angle — the corner of a book, a piece of paper, a window. Hold the corner up to the angle you are checking. If they match, the angle is right. If it falls inside the corner, the angle is acute. If the corner falls inside the angle, it is obtuse.

We usually mark a right angle with a small square at the vertex, not a curved arc.

Quick checks without a protractor

• Is the angle smaller than a square corner? → acute.
• Does the angle exactly match a square corner? → right.
• Is the angle bigger than a square corner but you can still see two separate arms? → obtuse.
• Are the two arms in a perfect line? → straight.

Less common names (you'll hear them later)

• A reflex angle is bigger than 180° but less than 360° (the "outside" of an obtuse angle).
• A full angle is exactly 360° — one complete spin back to the start.

You don't have to use these names in Year 4, but it's good to know they exist.

What's next

• Using a protractor
• Angles as fractions of a turn
• Back to the introduction

Try it out

• 🔢 Classify the angle — acute, right or obtuse
• 📐 Measure with a protractor — read the exact degrees