Grade 4 geometry lesson
How to Measure Angles with a Protractor: Steps, Examples, and Practice
To measure an angle with a protractor, place the center on the vertex, line up 0° with one ray, and read where the other ray crosses the scale.
What does a protractor do?
A protractor is a tool for measuring angles.
It measures how wide an angle opens.
The answer is written in degrees, using the degree symbol: °.
The protractor rule
Use this rule every time you measure an angle.
Put the center of the protractor on the vertex, line up 0° with one ray, then read where the other ray crosses the scale.
The vertex is the corner point where the two rays meet.
Step-by-step guide
Step 1: Find the vertex. This is the point where the two rays meet.
Step 2: Put the small center mark of the protractor exactly on the vertex.
Step 3: Turn the protractor so the 0° line sits on one ray.
Step 4: Follow the other ray to the curved scale.
Step 5: Read the number where the ray crosses the scale. That number is the angle measure.
Parts of a protractor
Most classroom protractors are shaped like half a circle.
The straight bottom edge helps you line up one ray.
The small center mark goes on the vertex. The curved scale shows degrees from 0° to 180°.
Which scale should you read?
Many protractors have two number scales.
Start reading from the 0° that is lined up with the ray.
If your ray starts on the right side, use the scale that begins at 0° on the right. If your ray starts on the left side, use the scale that begins at 0° on the left.
Worked example
Problem: Measure an angle with one ray flat on the right and the other ray crossing the protractor at 55°.
Step 1: Put the center mark on the vertex.
Step 2: Line up the 0° mark with the flat ray.
Step 3: Read the number where the slanted ray crosses the scale.
Answer: The angle measures 55°.
Using the measure
After you measure an angle, you can name its type.
If the angle is less than 90°, it is acute. If it is exactly 90°, it is a right angle. If it is more than 90° but less than 180°, it is an obtuse angle.
Review types of angles if you want help matching each measure to its name.
Common mistakes
Do not put the protractor center beside the vertex. It must sit on the vertex.
Do not line up the bottom edge with the wrong place. The 0° line should sit on one ray.
Do not read the scale from the wrong zero. Start from the 0° that touches the ray.
Do not guess from the picture size. A longer ray does not make a bigger angle; the opening is what matters.
Quick practice
1. The center mark of the protractor goes on the vertex.
2. One ray should line up with 0°.
3. If the other ray crosses the scale at 40°, the angle measures 40°.
4. If the angle measures 100°, it is obtuse.
5. If the angle measures 90°, it is a right angle.
Interactive playground
Measure with a protractor
Choose an angle from 0° to 180°, then play the measuring motion from 0° to the ray.