Angle families
Acute is less than a right angle, obtuse is greater than a right angle, and reflex is greater than a straight line.
Estimate by comparing to a right angle or a straight line.
Year 5
Know angles are measured in degrees, estimate and compare acute, obtuse and reflex angles
Use degree language to estimate, compare and classify angles correctly.
Lesson overview
Geometry - properties of shape
Angles measure turn. We measure them in degrees. Acute angles are less than 90 degrees, right angles are 90 degrees, obtuse angles are more than 90 degrees but less than 180 degrees, and reflex angles are more than 180 degrees.
Compare by size
An angle of 120 degrees is greater than an angle of 65 degrees.
The larger degree measure shows the larger turn.
Use benchmark angles
A straight line is 180 degrees and a whole turn is 360 degrees.
These benchmark angles help when estimating.
Worked example
Classify an angle of 135 degrees.
- Compare 135 degrees with 90 degrees.
- It is greater than a right angle.
- Compare 135 degrees with 180 degrees.
- It is an obtuse angle.
Round 1: Learn it
Classify angles by comparing them to 90 degrees and 180 degrees.
Finish this round to unlock Round 2: Show you know it.
Question 1
Which word describes an angle of 45 degrees?
Question 2
Which word describes an angle of 135 degrees?
Question 3
Which angle is larger?
Question 4
How many degrees are in a straight line?
Question 5
Which word describes an angle of 220 degrees?
Common mistakes
Watch for these as you work through the lesson.
- Calling any angle over 90 degrees a reflex angle.
- Forgetting that a right angle is exactly 90 degrees.
- Comparing angles by side length instead of amount of turn.
Related topics
These ideas connect closely to this lesson.
- Draw given angles, and measure them in degrees.
- Identify angles at a point and one whole turn, on a straight line and at a point on a line, and use these facts to find missing angles.