How to measure angles with a protractor

An angle measures a turn, in degrees. To measure one, put the protractor’s center on the corner, line up 0 with one arm, and read where the other arm crosses the scale. This page explains degrees, the types of angles, how to use both protractor scales without mixing them up, and has an interactive angle maker.

What an angle measures

An angle is the amount of turn between two rays that meet at a point (the vertex). A full turn is 360°, a half turn is 180° and a quarter turn, a right angle, is 90°. The length of the arms doesn’t change the angle.

Types of angles

Acute (50°)Right (90°)Obtuse (130°)Straight (180°)
Acute (less than 90°), right (90°), obtuse (between 90° and 180°), straight (180°). Angles over 180° are reflex.

Angle maker

Drag the slider from 0° to 360°. The angle is drawn on a protractor and named.

How to use a protractor, step by step

  1. Estimate first: is the angle acute or obtuse? This tells you whether the answer is under or over 90°.
  2. Put the center point of the protractor exactly on the vertex.
  3. Line up the 0 line with one arm of the angle.
  4. Read the scale that starts at 0 on that arm, where the other arm crosses it.
  5. Check against your estimate. If you estimated acute and read 130°, use the other scale: 50°.

Arms too short to reach the scale? Extend them with a ruler first.

Adding angles

Angles that sit side by side add up (4.MD.C.7). If a right angle is split into two parts and one is 35°, the other is 90° − 35° = 55°. On a straight line the parts add to 180°, and all the way round a point they add to 360°.

Common mistakes

Read 130° for a 50° angleUsed the wrong scale.Fix: estimate acute or obtuse first, then use the scale that starts at 0 on your arm.
Center not on the vertexThe protractor was shifted.Fix: line up the small center mark exactly on the corner.
Thinks longer arms make a bigger angleJudged by size.Fix: only the turn matters, not the arm length.
Started from 1 not 0Like a ruler mistake.Fix: one arm must sit on the 0 line.

How to teach it at home

Door angles

Open a door to acute, right and obtuse angles.

Clock hands

What angle do the hands make at 3:00? (90°) At 6:00? (180°)

Paper corner tester

Use a sheet of paper’s corner to check for right angles around the house.

By grade

Worksheets and practice

Types of angles worksheet, page 1

Types of angles worksheet: printable PDF with an answer key. · Make a fresh test

Angles chart · Angle relationships (PDF) · Geometry

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Questions parents ask

How do you measure an angle with a protractor?

Put the center point on the vertex, line up 0 with one arm, and read the scale that starts at 0 where the other arm crosses it.

Which scale do I use on a protractor?

The one that starts at 0 on the arm you lined up. Estimate first: acute angles are under 90°, obtuse over 90°.

What are the types of angles?

Acute (under 90°), right (90°), obtuse (between 90° and 180°), straight (180°) and reflex (over 180°).

How many degrees are in a full turn?

360°. A half turn is 180° and a quarter turn is 90°.

What grade learns to measure angles?

Grade 4 (4.MD.C.6).