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

What is an acute angle?

An acute angle is an angle that opens a small amount.

It is smaller than a square corner, but it is still more than a zero angle.

In degrees, an acute angle is more than 0° and less than 90°.

Acute angle rule

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

0° < acute angle < 90°

So 1°, 35°, and 89° are acute. But 0° is a zero angle, and 90° is a right angle.

How acute angles compare

Compare an acute angle with nearby angle names.

An acute angle is smaller than 90°. A right angle is exactly 90°. An obtuse angle is more than 90° but less than 180°.

For the full list, review types of angles.

Everyday examples

A door opened just a little can show an acute angle.

The tip of a sharp pencil can look like an acute angle.

On a clock, the smaller angle between the hands can be acute at some times, such as about 1 o clock.

Worked example

Problem: Is 38° an acute angle?

Step 1: Check if the angle is more than 0°.

Step 2: Check if the angle is less than 90°.

Step 3: 38 is more than 0 and less than 90.

Answer: 38° is an acute angle.

Acute angle and acute triangle

An acute angle is one angle.

An acute triangle is a triangle that has three acute angles inside it.

A right angle is exactly 90°, so a triangle with a right angle is not an acute triangle.

Common mistakes

Do not call 0° acute. An acute angle must open more than 0°.

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

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

Quick practice

1. 25° is acute.

2. 90° is not acute because it is a right angle.

3. 89° is acute.

4. 120° is not acute because it is more than 90°.