Put the centre of the protractor on the corner of the angle and line the baseline up with one arm. That arm points to 0 on one of the two scales. Follow that same scale round to the other arm and read the number. Then check: if the angle is smaller than a right angle, the answer must be less than 90. If neither arm is on 0, read both arms on the same scale and find the gap between the numbers.
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
- protractor
- a tool for measuring angles in degrees
- scale
- a row of numbers on the protractor; there is an inner and an outer one
- degree
- the unit for measuring turn, written °
Worked examples
One arm is on 0 of the inner scale; the other points to 60 on the inner scale (120 on the outer). How big is the angle?
- 1.Start at the 0 under the first arm: that is the inner scale.
- 2.The angle is smaller than a right angle, so it must be under 90.
- 3.Read 60 on the inner scale.
= 60
One arm is on 0 of the outer scale; the other points to 135 on the outer scale. How big is the angle?
- 1.Start at the 0 on the outer scale.
- 2.The angle is bigger than a right angle, so it is more than 90.
- 3.Count the small mark after 130: 135.
= 135
The arms point to 30 and 110 on the inner scale. How big is the angle?
- 1.Neither arm is on 0, so find the gap.
- 2.110 − 30 = 80.
= 80
For parents
Method: Measure angles in the house together and always do the 'smaller or bigger than a right angle?' check.
What to say: Which arm is on 0? Which row of numbers starts there? Does your answer make sense for the size of the angle?
Common mistakes:
- Reads the number on the wrong scale of the protractor, giving 180° minus the angle.
- When neither arm is on 0, gives the reading under one arm instead of the gap between the two readings.
- Ignores the small unnumbered marks and reads the last numbered mark before the arm.