Grades Grade 4 Measurement & DataMeasuring Angles with a Protractor

Measuring Angles with a Protractor

📐 Measurement & Data

📐

Measuring Angles with a Protractor

Angles are measured in degrees — and a protractor is your angle ruler!

An ANGLE is formed by two rays that share the same endpoint (the vertex). We measure angles in DEGREES (°). A full rotation is 360°. A protractor is a half-circle tool marked from 0° to 180° used to measure and draw angles.

Types of angles:

Acute angle: less than 90°Right angle: exactly 90° (square corner)Obtuse angle: between 90° and 180°Straight angle: exactly 180° (a straight line)Reflex angle: between 180° and 360°

A right angle marker (small square) is drawn in the corner of 90° angles.

How to measure an angle with a protractor:

1Place the center hole of the protractor exactly on the VERTEX of the angle

2Line up the baseline (0° line) with one ray of the angle

3Read the scale where the other ray crosses the protractor

4Use the INNER scale if you started from the right; OUTER scale if from the left

Acute (< 90°)

30°45°60°75°

Obtuse (90° – 180°)

120°135°150°165°

A right angle (90°) is the boundary between acute and obtuse.

🔄Additive angles

Two angles share a common ray and together form a larger angle.
If one part measures 35° and the other measures 55°,
the combined angle = 35° + 55° = 90° (a right angle!).

\angle ABC = \angle ABD + \angle DBC

💡

Remember This!

Before measuring, ESTIMATE: Is the angle less than 90° (acute) or more than 90° (obtuse)? This helps you pick the right scale on the protractor and avoid reading the wrong number.

✏️ Try It!

An angle measures 127°. What type of angle is it?

Take Quiz 📝 — 25 Questions