A straight angle is an angle that measures exactly 180°.
It forms a straight line, meaning the two rays extend in opposite directions from a common point.
∠RAT is a straight angle.
Look at the angles below. Which is a straight angle?
∠PIG is a straight angle.
Finding Straight Angles in the Real World
Straight angles can be found all around us! Unlike acute angles, which are small and narrow, or obtuse angles, which are wide and open, straight angles form a perfectly straight line.
They appear in many everyday objects, especially when two parts of an object extend in opposite directions.
Here are some fun examples:
- The edge of a ruler – A ruler’s straight edge forms a 180° angle, making it a perfect example of a straight angle.
- A clock showing 6:00 – The hour and minute hands are in a perfectly straight line.
- A balance scale forms a straight angle when both sides are perfectly even. If one side moves up or down, the angle changes, but when balanced, it’s exactly 180°.
A straight angle represents a complete half-turn. If you rotate exactly 180°, you’ll end up facing the opposite direction from where you started.
Can you spot any straight angles in the world around you?
Measuring a Straight Angle with a Protractor
You can measure angles using a tool called a protractor.
A standard protractor measures angles from 0° to 180°, which means it can be used to measure a straight angle perfectly. Since a straight angle always measures exactly 180°, it looks like a straight line.
On a protractor, the tick marks between each 10° increment represent single degrees (1°).
When measuring a straight angle, you’ll see that one ray lines up with 0° and the other extends all the way to 180°, confirming that it is a straight angle.
Full-circle protractors measure angles all the way from 0° to 360°.
Not all protractors stop at 180°! Some full-circle protractors measure angles all the way from 0° to 360°. This allows you to measure angles larger than 180°, including reflex angles (angles between 180° and 360°).
A straight angle splits a full-circle protractor into two equal halves—each side measuring 180°.
When you add both halves together, they form a full 360° circle!
Practice Problems
Which is a straight angle?
What is the measure of a straight angle?
Which of these objects is most likely to show a straight angle?
A straight angle is smaller than a right angle.
A right angle is 90°, and a straight angle is 180°, which is larger.
