Using a Protractor
Measuring angles with a protractor can be a confusing experience. There are many things to which you must pay attention. Understanding that angles are parts of circles is helpful, because then the design of the protractor makes sense. With a little practice, you will soon be measuring angles with ease.
By the end of this lesson, your children will be able to use a protractor to measure and draw angles of various sizes.
Recall what you have learned about angles already. Angles are sections of a full circle. Since a full circle measures 360º, an individual angle has to measure less than 360º, because it is part of a circle.
Estimating the approximate measurement of a circle before you actually measure it can be helpful. By comparing your actual answer to your estimated measurement, you can determine whether or not your answer is reasonable.
In this lesson you will estimate angle measurements, find actual measurements using a protractor, and draw angles of various sizes.
Have your children take the Pre-Test below to see if they are ready for this lesson. If they get 7 or less correct, review degrees in a circle with them before continuing on to the lesson.
- Estimating Angles - Pre-assessment
Main Lesson: Using a Protractor
Now it's time to talk about the tool that is used to measure angles. You may have wondered why it is shaped the way it is. Knowing that angles are measured by how many degrees of a circle are between the two rays, the shape starts to make more sense.
Help your children see the similarities between a half circle and the protractor. Show them that two protractors with the straight edges together would form a whole circle. Point out the similarities between the measurement markings and a regular ruler.
Line up the endpoint of the rays, called the vertex of the angle, with the crossed lines in the middle of the straight edge on the protractor. Line up the line that points to zero lined up with one of the rays that make the angle. Use the ruler on the curved edge to count the degrees until you reach the other ray in the angle.
Now that you understand how a protractor works, let's look at a couple of angles and measure them:
Sometimes the angle will be smaller than the protractor. Use the straight edge to extend the ray to make it easier to read the measurement. The second ray on this angle points to 45, so this is a 45° angle.
Lining up the vertex with the crossed lines, and the first ray with the line pointing to zero, we extend the second ray to clearly see that it is pointing to 138. This angle measures 138°.
Whatever number of one-degree curves is an angle, that number equals the measurement of the angle.
Remind your children that it is very important to line up the protractor correctly so they can get a correct measurement. The crossed lines at the bottom of the straight edge must be matched up with the vertex (endpoint) where the two rays are joined.
A protractor can also be used to draw angles.
1. Begin by using the protractor’s straight edge to draw the first ray.
2. Line up the endpoint of the ray with the crossed lines on the straight edge of the protractor. Follow the numbers on the curve and make a mark by the number of the angle you want to draw.
3. Use the straight edge to connect the mark with the endpoint of the first ray.
4. Label the angle with the correct measurement.
- Angles are sections of a circle.
- Angles are measured in degrees.
- There are 360° in a circle.
- Individual angles measure less than 360°
- A protractor is used to measure angles.
- A protractor can also be used to draw an angle.
- It is important to line up the protractor correctly.
Review the above recap points with your children and then print out the Post Test that follows.
- Measuring Angles Using a Protractor - Post-assessment
At least 7 out of 10 correct will show that your children are ready to go on to the next lesson: Add or Subtract to Find Unknown Angles.