What is an angle?
An angle is formed when two straight lines (or two line segments) start from the same point, called the vertex. It is measured in degrees (°), a bit like how you measure a length in centimetres. A full turn is 360°.
The 4 types of angle
Right angle: exactly 90°
Acute angle: smaller than 90°
Obtuse angle: between 90° and 180°
- Acute angle: smaller than a right angle (less than 90°).
- Right angle: exactly 90° — the two lines are perpendicular.
- Obtuse angle: bigger than a right angle, smaller than a straight angle (between 90° and 180°).
- Straight angle: exactly 180° — the two lines actually form a single straight line.
Using a protractor
A protractor is a graduated half-circle from 0° to 180°, used to measure or draw an angle with precision.
- I place the centre of the protractor (the small mark in the middle of the base) exactly on the vertex of the angle.
- I line up the baseline (the 0°) with one of the two sides of the angle.
- I read the scale crossed by the other side of the angle: that's the measurement in degrees.
Recognising a right angle without a protractor
Handy trick: the corner of a sheet of paper or a set square forms a perfect right angle. To check an angle, lay the corner over it: if it lines up exactly, it's a right angle; if there's a gap left over, the angle is acute; if the corner overshoots, the angle is obtuse.
Mistakes to avoid
- Placing the centre wrongly (not exactly on the vertex).
- Reading the wrong scale (the two series running from 0 to 180°).
- Confusing an obtuse angle with a straight angle — a straight angle forms a complete straight line.
- Assuming that an angle which "looks big" on paper must be obtuse: the length of the lines makes no difference to the angle's measurement — only the amount of turn matters.
Worked exercises
An angle measures 45°. What type is it?
An angle measures 120°. What type is it?
How many degrees are in a right angle?
Now it's your turn!
On EasyMaths, your child practises angles with exercises that never repeat and instant marking. The first exercises are free.
Start the exercises →Frequently asked questions
How do you recognise a right angle without a protractor?
You can use the corner of a set square or a sheet of paper: if the corner lines up exactly with the angle, it's a right angle (90°).
How do you use a protractor?
Place the centre of the protractor on the vertex of the angle, the baseline (0°) on one of the sides, then read the scale crossed by the other side of the angle.
What is the difference between an acute angle and an obtuse angle?
An acute angle is smaller than a right angle (less than 90°). An obtuse angle is bigger than a right angle but smaller than a straight angle (between 90° and 180°).