Protractor & Angle Practice
A real protractor on your screen — with both the inner and the outer scale. Slide it onto the vertex, line the baseline up with an arm, and learn to read the correct scale. Then draw angles, learn every rule and test yourself.
To measure an angle, put the protractor's centre on the vertex, lay its baseline along one arm, and read where the other arm crosses the scale that starts at 0 on that arm. The two scales always add to 180°, so if one reads 65° the other reads 115°.
Place the protractor
Drag the protractor by its body. Use the round knob on the curve to turn it.
The angle
A new angle every time — in any direction, like in a real question.
What the protractor says
Line the protractor up to get a reading.
How to measure an angle
- Put the centre point of the protractor exactly on the vertex — the corner where the two arms meet.
- Turn the protractor so its baseline (the 0 line) lies along one arm.
- Find where the other arm crosses the curved edge.
- Read the scale whose 0 is on the arm you lined up. The other scale gives 180° minus your angle — that is the usual mistake.
Set the angle
Type it or slide it — from 0° right round to 360°.
Pairs of this angle
Drawing it by hand
The six types of angle
Tap any one to draw it in the Draw tab.
Find the missing angle
Pick the rule, type the angles you know, and the missing one appears with the working.
Reading a protractor the right way
Most marks lost on angles are not drawing mistakes — they are reading the wrong one of the two scales. This tool shows both scales at once so you can see which to use and why.
Centre on the vertex
The small cross in the middle of the flat edge goes exactly on the corner.
Baseline on an arm
Turn the protractor until its 0 line lies along one of the two arms.
Follow the other arm
See where it crosses the curved scale.
Pick the right scale
Read the scale whose 0 sits on the arm you lined up.
Why two scales?
A protractor is printed with two sets of numbers so it can be used from either end. At every mark the two numbers add up to 180:
inner reading + outer reading = 180°
So if one scale says 65° the other says 115°. Choose the one that starts at 0 on your lined-up arm. If the answer looks far too big or too small for the shape of the angle, you have read the wrong scale.
Naming angles
- Acute: less than 90°
- Right: exactly 90°
- Obtuse: between 90° and 180°
- Straight: exactly 180°
- Reflex: between 180° and 360°
- Complete: exactly 360°
Angle pairs
Complementary angles add to 90°. Supplementary angles add to 180°.
a + b = 90° or a + b = 180°
Angles on a straight line add to 180° (a linear pair), angles round a point add to 360°, and when two straight lines cross, the vertically opposite angles are equal.
Drawing an angle of 65°
- Draw a ray OA with a ruler.
- Put the centre of the protractor on O with the baseline along OA.
- Count from the 0 that sits on OA up to 65 and make a dot.
- Join O to the dot and label it B. ∠AOB = 65°.
For a reflex angle such as 290°, draw 360° − 290° = 70° instead and mark the angle on the outside.
Protractor & Angle Practice — your questions
How do you use a protractor to measure an angle?
Put the centre mark on the vertex, line the zero edge along one arm, then look at where the other arm crosses the curved scale. Read the scale whose zero sits on the arm you lined up. If the arm is too short, extend it with a ruler before reading.
Why does a protractor have two sets of numbers?
So it can be used from either end, whichever way the angle opens. At every mark the inner and outer readings add to 180°, so where one says 65° the other says 115°. Choose the scale that starts at 0 on your lined-up arm, and check the answer against the shape of the angle.
What are the types of angles?
An acute angle is less than 90°, a right angle is exactly 90°, an obtuse angle lies between 90° and 180°, a straight angle is exactly 180°, a reflex angle lies between 180° and 360°, and a complete angle is 360°. Deciding the type by eye first stops you reading the wrong scale.
What is the difference between complementary and supplementary angles?
Two angles are complementary when they add to 90° and supplementary when they add to 180°. The complement of 35° is 55°, and its supplement is 145°. Angles that sit next to each other on a straight line form a linear pair, so they are always supplementary.
How do I draw an angle of 65 degrees?
Draw a ray OA with a ruler. Put the centre of the protractor on O with the baseline along OA. Count from the zero that sits on OA up to 65 and mark a dot. Join O to the dot and label it B, so angle AOB = 65°. Check by measuring it again.
How do you measure a reflex angle?
A protractor only reads up to 180°, so measure the smaller angle on the other side and subtract it from 360°. If the inner angle measures 70°, the reflex angle is 360° − 70° = 290°. To draw a reflex angle of 290°, draw 70° instead and mark the angle on the outside.