Types of Angles

What Angle Types Describe

Angle type names describe the size of a turn. A zero angle is closed, an acute angle is smaller than a right angle, a right angle is exactly 90 degrees, an obtuse angle is larger than 90 but smaller than 180, a straight angle is 180 degrees, and a reflex angle continues beyond a straight line.

The fastest way to classify an angle is to compare it with 90 and 180 degrees before reading exact tick marks. If it opens less than a square corner, it is acute. If it opens past a square corner but not to a straight line, it is obtuse. If it passes 180 degrees, it is reflex.

Angle Type Explorer

Use Angle Maker to drag a ray through common examples and watch the type label change.
Check boundary values carefully: 0 degrees is not acute, 90 degrees is not acute or obtuse, 180 degrees is straight, and 360 degrees returns to a full turn.
Use the online protractor workspace when a type must be verified from an image, PDF, screenshot, or diagram rather than a clean generated example.

Degree Ranges

Zero angle: exactly 0 degrees, with both rays pointing in the same direction.
Acute angle: greater than 0 degrees and less than 90 degrees.
Right angle: exactly 90 degrees, like the corner of a square.
Obtuse angle: greater than 90 degrees and less than 180 degrees.
Straight angle: exactly 180 degrees, forming a straight line.
Reflex angle: greater than 180 degrees and less than 360 degrees.
Full angle: exactly 360 degrees, one complete turn.

Common Confusions

A 90 degree angle is right, not acute or obtuse, even if the drawing looks slightly rounded.
A 180 degree angle is still an angle because it represents a half turn, although it looks like a line.
A reflex angle is not the same as a negative angle. Negative notation describes direction; reflex describes a size greater than 180 degrees.
Angles can be written beyond 360 degrees in rotation problems, but their basic type usually uses the equivalent angle within one full turn.
Estimate first, then measure. A quick comparison with 90 and 180 degrees helps catch reversed protractor readings.

Using an Online Protractor to Verify Types

When an angle comes from a real image or scanned diagram, classify it after checking point placement. A small visual error near 90 or 180 degrees can change the label. Use generated examples for learning the categories and the measurement workspace for checking a source file.

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

Frequently Asked Questions

Is 90 degrees acute or obtuse?

Neither. An angle of exactly 90 degrees is a right angle. Acute angles are smaller than 90 degrees, and obtuse angles are larger than 90 degrees but smaller than 180 degrees.

Is 180 degrees an angle?

Yes. A 180 degree angle is called a straight angle because the two rays point in opposite directions and form a straight line. It represents a half turn.

What is the difference between a reflex angle and a negative angle?

A reflex angle describes size: more than 180 degrees and less than 360 degrees. A negative angle describes direction of rotation. The same final ray can sometimes be described with either convention, depending on the lesson.

Can an angle be greater than 360 degrees?

In rotation problems, yes. A value greater than 360 degrees means more than one full turn. For basic angle type classification, reduce it to the equivalent angle within one 0 to 360 degree turn.

How can I estimate an angle before measuring it?

Compare it with familiar landmarks. Less than a square corner is acute, a square corner is right, past a square corner but before a line is obtuse, a line is straight, and beyond a line is reflex.