Measuring angles with a protractor, step by step
Reading a protractor is two skills a child learns at once.
Measure angle ABC.
Measure an angle
- Put the protractor's ORIGIN — the little cross or hole in the middle of its straight edge — exactly on the vertex B. Not the corner of the tool, which is the commonest slip.
- Turn the protractor so its zero line lies along one arm. The angle has not changed by turning the tool — you are choosing which arm to count from.
- Read the scale that starts at ZERO on that arm. A protractor prints two scales running opposite ways, so the same mark is 60 on one and 120 on the other; the one that reads zero where your arm is, is yours.
- Check the reading against the shape before you write it: a wide-open angle cannot be 40°, and a narrow one cannot be 140°. This one is 25°.
How it works.
- 01
Put the protractor's ORIGIN — the little cross or hole in the middle of its straight edge — exactly on the vertex B. Not the corner of the tool, which is the commonest slip.
- 02
Turn the protractor so its zero line lies along one arm. The angle has not changed by turning the tool — you are choosing which arm to count from.
- 03
Read the scale that starts at ZERO on that arm. A protractor prints two scales running opposite ways, so the same mark is 60 on one and 120 on the other; the one that reads zero where your arm is, is yours.
- 04
Check the reading against the shape before you write it: a wide-open angle cannot be 40°, and a narrow one cannot be 140°. This one is 25°.