An angle measures the amount of turn between two arms, not how long the arms are. Two arms can be long or short — what matters is how far one is turned from the other. We measure the turn in degrees with a protractor. Line one arm up with the 0° mark and read the scale that starts from there. An acute angle is less than 90°, so its measurement must be a small number. Watch the two rows of numbers on a protractor: follow the same row you started from, or you will read the angle's supplement by mistake.
Explore this idea with maths tools
Maths you can move
Build 8 + 5
Tap two gold counters to fill the ten-frame.
8 in the frame + 5 waiting. The total stays the same.
Words to know
- degree
- the unit for measuring an angle
- acute angle
- an angle smaller than a right angle (under 90°)
- protractor
- the tool used to measure angles
Worked examples
One arm on 0°, the other on the 40° mark. What is the angle?
- 1.Follow the scale from the 0° arm.
- 2.It is acute, so the answer is 40°.
= 40
An acute angle points to the 55° mark. How big is it?
- 1.Read the same scale as the starting arm.
- 2.The angle is 55°.
= 55
Arms 8 cm long, open arm on the 30° mark. How big is the angle?
- 1.Arm length is not the angle.
- 2.The turn measures 30°.
= 30
For parents
Method: Open a pair of scissors to show turn is separate from arm length.
What to say: Say 'the blades are the same length — what changes is how far they open'.
Common mistakes:
- Reading the wrong protractor scale (40° read as 140°).
- Giving the arm length in cm instead of the angle in degrees.