Grade 4 Geometry: Measuring Angles
Use a protractor
What You'll Learn
- Read a protractor
- Measure angles
- Add and subtract angle measures to find a whole angle or a missing part (4.MD.C.7)
📖Measuring
Line up the vertex with the center! Put one ray on a 0° mark, then read the number where the other ray crosses the SAME scale: the outer scale when 0° is on the right, the inner scale when 0° is on the left.
💡Neither ray on 0°?
Read both rays on one scale and subtract: rays at 30° and 110° make a 110 − 30 = 80° angle.
✏️Angles add up
An angle split into two parts that do not overlap measures the sum of the parts: 35° + 50° = 85°. To find a missing part, subtract: a 90° right angle with a 25° part leaves 90 − 25 = 65°.
Worked examples
Real problems from the practice worksheets, solved one step at a time.
Example 1
What is the measure of the angle on the protractor?
- One ray lies on the 0° mark on the right, so read the outer scale (it starts at 0 on the right).
- Follow the other ray out to the outer scale: it points to 160.
- The angle measures 160°.
Answer: 160
Example 2
Neither ray is on 0°. What is the measure of the angle on the protractor?
- Neither ray is on 0°, so read both rays on the same scale (the outer one, which starts at 0 on the right).
- One ray points to 10 and the other to 135.
- The angle is the difference: 135 − 10 = 125.
- The angle measures 125°.
Answer: 125
Example 3
Angle ABC is split into two angles that do not overlap. One measures 30° and the other measures 50°. What is the measure of angle ABC?
- The two smaller angles fill angle ABC with no overlap, so add their measures.
- 30° + 50° = 80°.
- Angle ABC measures 80°.
Answer: 80