Naming and Measuring Angles

An angle smaller than a right angle is acute. One bigger, but less than a straight line, is obtuse.

Straight angles

Exactly 180 degrees makes a straight line. The two arms point opposite ways.

Reading the scale

Line the vertex up with the centre and one arm with the zero. Read where the other arm crosses.

Estimate before reading

Decide first whether it is more or less than 90. That catches a reading off the wrong scale.

The four names

An acute angle is less than 90°. A right angle is exactly 90°. An obtuse angle is between 90° and 180°. A straight angle is exactly 180°. The names describe ranges, so classifying an angle means comparing it with 90 and 180.

Using a protractor

Put the centre point on the vertex and one arm along the zero line. Read where the other arm crosses the scale. The centre must sit exactly on the vertex — a protractor placed slightly off gives an answer that looks perfectly reasonable and is wrong.

Which of the two scales to read

Most protractors have two sets of numbers running opposite ways, so every position shows two readings, like 60 and 120. Decide first whether the angle is acute or obtuse, then take the reading that matches. This is the single most common protractor error.

Estimate before measuring

Look at the angle and judge it against 90° first. If it is clearly less than a right angle and your protractor says 130°, you read the wrong scale. The estimate takes a second and makes the measurement self-checking.