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Angle Measurement and Classification

5 min readGrade 6 · Geometry

A right angle is exactly 90°. An acute angle is smaller (like 45°). An obtuse angle is bigger (like 120°). A straight line is 180°, and a reflex angle wraps around past that.

Angle classification

Acute: 0° < angle < 90°. Right: exactly 90° (often marked with a small square). Obtuse: 90° < angle < 180°. Straight: exactly 180° (a straight line). Reflex: 180° < angle < 360° (angles that 'wrap around').

Measuring with a protractor

Align the center of the protractor at the vertex, one ray along the 0° line, and read where the other ray crosses. Be careful: protractors have two scales (inner and outer). Pick the right one based on your estimate.

Using reference angles

Know that 45° is a quarter of a right angle (half of 90°). A right angle is 90°. A straight line is 180°. Use these to estimate before measuring. This builds angle sense.

KEY VOCABULARY

Acute angle — An angle smaller than 90°.

Right angle — An angle of exactly 90°.

Obtuse angle — An angle between 90° and 180°.

Straight angle — An angle of exactly 180°.

Reflex angle — An angle between 180° and 360°.