Estimating Angles
Estimate and compare acute, obtuse, and reflex angles in degrees; classify angles by type and order them by size
What a learner can do afterwards
- Estimate an angle as approximately 130° and classify it as obtuse
- Identify a reflex angle in a diagram and estimate it as about 270°
- Order four angles from smallest to largest by estimation before measuring
The lesson
You already know a full turn is 360 degrees and a quarter turn is 90 degrees. Estimating an angle means looking at it and guessing its size in degrees, before you check with a protractor. The best trick is to compare the angle to a right angle, which is exactly 90 degrees.
Angles have names based on their size. An acute angle is smaller than 90 degrees, like a narrow wedge. An obtuse angle is bigger than 90 degrees but smaller than 180 degrees, like a door opened wide. A reflex angle is bigger than 180 degrees, more than half of a full turn.
Priya looks at an angle that seems a bit wider than a right angle, maybe by a third again. She guesses about 130 degrees and calls it obtuse, since it's more than 90 degrees but less than 180. When she checks with a protractor, it reads 128 degrees. Her guess was close.
To order angles from smallest to largest without measuring, sort by type first: acute angles come before obtuse angles, and reflex angles always come last, since they pass the halfway point of a full turn.
Compare each angle to a right angle: acute is under 90 degrees, obtuse is between 90 and 180, and reflex is over 180.
Watch it
Where it sits
Learn first
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Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.