Types of Angle: Acute, Right, Obtuse + How to Measure

There are five types of angle: acute (under 90°), right (exactly 90°), obtuse (between 90° and 180°), straight (exactly 180°) and reflex (between 180° and 360°). A full turn is 360°. Angles start in Year 4 and protractor measuring in Year 5.

Measuring with a Protractor

Put the protractor centre on the vertex, line one arm up with zero, and read the scale that starts at zero on that arm. Estimate first: an obviously acute angle cannot read 130°, so the answer is 50°.

Relationships

Supplementary angles add to 180°, complementary angles add to 90°, and vertically opposite angles are equal. Angle sums: triangle 180°, quadrilateral 360°, n-gon (n − 2) × 180°.

Clock Angles

Each hour is 30° and each minute on the long hand is 6°. At 3:30 the angle is 75°, and at 2:40 it is 160°.

Worked Examples

• Triangle with 50° and 60°: third angle 70°.
• Supplementary angles where one is double the other: 60° and 120°.
• Complement of 35°: 55°.

Summary

Most errors come from reading the wrong protractor scale. Algonova has helped more than 1,000,000 students in 97 countries, with a free maths diagnostic of about 60 minutes.