Angle Type Explainer
Learn about acute, right, obtuse, straight, and reflex angles. See what type any angle is.
An angle is the amount of turn between two lines (or rays) that meet at a point (the vertex). We measure angles in degrees (°). A full turn is 360°—one complete circle. Drag the line below to make an angle, or type a number. Scroll down to learn each type.
Draw an angle
Move the blue handle to change the angle. The fixed line stays horizontal; the other line follows your drag.
45.0°
Drag the blue handle to change the angle
45.0°→Acute angle
Or type an angle
Enter an angle in degrees (0 to 360). The diagram above will update, and we’ll show the type.
Angle types
From smallest to largest turn:
Acute angle
Greater than 0°, less than 90°
A sharp, narrow angle. Think of the corner of a slice of pizza or the hands of a clock at 2 o’clock.
Right angle
Exactly 90°
A square corner. The corner of a book, a piece of paper, or the letter L. A right angle is often marked with a small square.
Obtuse angle
Greater than 90°, less than 180°
Wider than a right angle but less than a straight line. Like the hands of a clock at 4 o’clock.
Straight angle
Exactly 180°
A straight line. Two rays pointing in opposite directions. The angle on a flat line.
Reflex angle
Greater than 180°, less than 360°
Bigger than a straight angle—the “other” part of the turn. If you turn past 180°, you’re in reflex territory.
Full turn (or zero)
Exactly 360° (or 0°)
A full rotation. You’ve turned all the way around back to the start. One complete circle.
Angles in daily life and simple facts
Angles are everywhere. Here are a few ideas and examples.
Angles you see every day
- • The corner of a book or a piece of paper is a right angle (90°).
- • A clock at 3 o’clock: the hands make a right angle. At 12 o’clock they make 0° (or 360°).
- • A slice of pizza is often an acute angle—less than 90°. Half a pizza is a straight angle (180°).
- • When you turn from facing north to facing east, you turn 90°. A full about-turn is 180°.
Complementary and supplementary
Two angles are complementary if they add up to 90°. For example, 30° and 60° are complementary. Two angles are supplementary if they add up to 180°. For example, 70° and 110° are supplementary. If two angles sit on a straight line, they are supplementary.
Angles in a triangle
The three angles inside any triangle always add up to 180°. So if you know two angles, you can find the third. Example: in a triangle with angles 40° and 60°, the third angle is 180° − 40° − 60° = 80°.
Naming an angle
An angle is often named by the vertex and the two rays, e.g. “angle ABC” or ∠ABC, where B is the vertex. You can also say “the angle at B.” The size of the angle doesn’t depend on how long the lines are—only how much one line is turned from the other.
Quick recap
- • Acute < 90°
- • Right = 90°
- • Obtuse between 90° and 180°
- • Straight = 180°
- • Reflex between 180° and 360°
- • Full turn = 360°
Quick questions
- Why 360° for a full circle?
- It’s an old convention. One idea is that it’s roughly the number of days in a year, so the Sun moves about 1° per day along the sky. Another is that 360 has many divisors (2, 3, 4, 5, 6, 8, 9, 10…), so it’s handy for splitting circles into equal parts.
- What’s the difference between 0° and 360°?
- For the amount of turn they’re the same—no turn vs a full turn. We usually say 0° when we mean “no angle” or the starting position, and 360° when we mean “one full rotation.”
- How do I measure an angle with a protractor?
- A protractor is a semicircular (or circular) tool marked in degrees. Line up its centre with the vertex of the angle and the 0° mark with one arm. Read the scale where the other arm crosses—that’s your angle. Practise with the diagram above: drag the handle and see how the angle changes.