Idea 5of 24Builds onA degree is a tiny turn
Benchmark angles for estimating
How can you judge an angle's size before you measure it?
Today you'll discover
- four angles that are easy to picture
- how to find the two an angle falls between
- how a good estimate catches mistakes
Think firstAbout how big is this angle?
Without a protractor, which is the best guess for this angle?
Decide first, then press Next. Compare the arm with the dashed lines.
An angle opening from a flat arm on the right. Dashed lines then fan out from its corner at 45°, 90°, 135° and 180°. The two dashed lines either side of the arm, 45° and 90°, are drawn darker with the band between them shaded, and the angle is measured as 60°. The last step shows a wider angle of 130°.
See the whole ideathe big idea and the key word, in one place
The big idea
Benchmark angles of 45°, 90°, 135° and 180° help you estimate any angle.
The four benchmarks
45° is half a square corner. 90° is a square corner. 135° is a square corner and a half. 180° is a straight line.
- Half corner45°
- Square corner90°
- And a half135°
- Straight180°
Easy to get wrongAn estimate is a check, not the answer. Measure to find the size; use the estimate to catch a wrong reading.
Key words
- benchmark angle
- an angle that is easy to picture, used to compare others: 45°, 90°, 135° or 180°
- estimate
- a careful guess at a size, made before measuring
Ready to use it?
- Estimate an angle using benchmark angles.
- Say which two benchmarks an angle falls between.