Obtuse Angle: Definition, Degrees, Examples, Chart, and Practice

What is an obtuse angle?

An obtuse angle is an angle that opens wide.

It is bigger than a square corner, but it does not make a straight line.

In degrees, an obtuse angle is more than 90° and less than 180°.

Obtuse angle rule

Use this rule when you decide if an angle is obtuse.

90° < obtuse angle < 180°

So 91°, 120°, and 179° are obtuse. But 90° is a right angle, and 180° is a straight angle.

How obtuse angles compare

Compare an obtuse angle with nearby angle names.

An acute angle is smaller than 90°. A right angle is exactly 90°. An obtuse angle is between 90° and 180°.

For the full list, review types of angles.

Everyday examples

A laptop opened wide can show an obtuse angle.

A door opened more than a square corner can also make an obtuse angle.

On a clock, the smaller angle between the hands can be obtuse at some times, such as about 4 o clock.

Worked example

Problem: Is 132° an obtuse angle?

Step 1: Check if the angle is more than 90°.

Step 2: Check if the angle is less than 180°.

Step 3: 132 is more than 90 and less than 180.

Answer: 132° is an obtuse angle.

Obtuse angle and obtuse triangle

An obtuse angle is one angle.

An obtuse triangle is a triangle that has one obtuse angle inside it.

So the angle name tells the size of one corner. The triangle name tells what kind of triangle it is.

Common mistakes

Do not call 90° obtuse. 90° is exactly a right angle.

Do not call 180° obtuse. 180° is exactly a straight angle.

Do not judge only by the drawing size. Use the degree measure or compare the opening to a right angle.

Quick practice

1. 100° is obtuse.

2. 89° is not obtuse because it is less than 90°.

3. 150° is obtuse.

4. 180° is not obtuse because it is a straight angle.