How to Use a Protractor

Two Major ideas:1. How to correctly place the protractor2. How to correctly read the protractor

Protractor: A tool to

measure angles

Part of the Protractor

Inner Scale

Zero-EdgeCenter Mark

OuterScale

Vocabulary WordsZero Edge Center Mark

Inner Scale Outer Scale

Always the same level as the 0 degree mark

Always at the middle of the zero-edge

The numbers on the inner edge of the protractor; the scale goes from right to left.

The numbers on the outer edge of the protractor; the scale goes from left to right.

Measuring an Angle

1. Determine if the angle is acute or obtuse before using the protractor.

Acute, Right or Obtuse?• Right Angle – an angle whose

measure is 90˚.

• Acute Angle – an angle whose measure is greater than zero but less than 90˚.

• Obtuse Angle – an angle whose measure is greater than 90 but less than 180˚.

Measuring an Angle

2. Place the center mark of the protractor on the vertex of the angle.

Measuring an Angle

3. Rotate the zero-edge of the protractor to line up with one ray of the angle for the other ray of the angle to cross the protractor’s scale.

Measuring an Angle4. Read the measure of the angle of the ray that

crosses the protractor’s scale.

Measuring an AngleStep 1 Step 2

Step 3 Step 4

Determine if it’s acute or obtuse.

Place center mark on vertex of the angle.

Line up zero-edge.Read angle’s measure.• If acute: use scale with

numbers less than 90• If obtuse, use scale with

number greater than 90

Common Mistakes

1. Using the incorrect scale.2. The center mark is not on the vertex.– This usually happens after rotating the protractor.

Always check your centering when moving the protractor.

3. Counting the marks in the wrong direction.– Pay attention to whether the numbers are

increasing or decreasing.

How would you measure these angles?

How about this angle?