How to Use a Protractor

What a Protractor Measures

A protractor measures the size of a turn between two rays or line segments. The center mark belongs on the vertex, the baseline belongs on one side of the angle, and the scale tells how far the other side has turned. The tool does not measure line length.

Most school protractors have two number scales because an angle can open from the left or from the right. Start from the zero that lies on the side you aligned with the baseline, then follow that same scale until it reaches the other side of the angle.

Step-by-Step: Measure and Draw Angles

Place the center mark exactly on the vertex. If the side is short, extend it lightly with a ruler so it reaches the scale without changing direction.
Rotate the protractor until the zero line is aligned with one side of the angle. Read the inner or outer scale that starts at that zero, not the scale that starts on the opposite side.
To draw an angle, make a baseline, place the center on one endpoint, mark the desired degree value on the correct scale, then draw a ray from the endpoint through the mark.

Common Mistakes and Fixes

Reading the wrong scale is the most common error. Estimate acute or obtuse first so a 40 degree angle is not accidentally read as 140 degrees.
Do not align the protractor with the paper edge unless the angle side is actually parallel to that edge.
Keep the center mark on the vertex; even a small shift changes the reading.
Extend short sides with a straightedge instead of guessing where they meet the scale.
For reflex angles, measure the smaller interior angle first and subtract it from 360 degrees.

Using an Online Protractor on Images

A physical protractor touches paper or an object. The online protractor measures a visible angle in pixels from an image, screenshot, diagram, PDF page, or blank practice canvas.
Use the browser workspace when the angle is already captured in a file. Place points on the visible sides, adjust the transparent overlay if helpful, and export the marked result.
Use Angle Maker when you need to draw a clean demonstration angle, and use practice pages when students need repeated reading exercises.

Practice Example and Checks

Before trusting a reading, ask whether the angle should be acute, right, obtuse, straight, or reflex. Then check the vertex, baseline, and scale choice. In the online workspace, check whether photo perspective, scan skew, or unclear edges could affect the visible angle.

This guidance page can be read without an account. When reference links to browser tools, ordinary measurement files remain local to your browser unless you choose to export them.

Frequently Asked Questions

Which scale should I read on a protractor?

Read the scale whose zero is aligned with the side you used as the baseline. If that side starts at the left zero, read the scale that increases from the left. If it starts at the right zero, read the scale that increases from the right.

Why does the same tick show two numbers?

The two numbers support angles that open from opposite directions. One scale increases clockwise and the other increases counterclockwise. Your baseline choice tells which one is correct.

What if the angle sides are too short to reach the scale?

Extend the sides with a ruler or straightedge, keeping the same direction. Do not move the vertex or guess a curved path to the tick marks.

How do I measure an angle greater than 180 degrees?

Measure the smaller angle between the sides first, then subtract that value from 360 degrees to find the reflex angle. A full-circle protractor can also show reflex angles directly.

How can I check whether my reading is reasonable?

Estimate the type before reading. If the angle looks acute, the answer should be less than 90 degrees. If it looks obtuse, it should be between 90 and 180 degrees.