Using a Protractor (Measuring Angles)

Lesson Objective

In this lesson, we will learn how to measure angles using a protractor.

Study Tips

Tip #1

It is important to ensure the following when using a protractor:

The protractor's origin is at the vertex of an angle.
The protractor's base line is along the arm.
Make sure that you are using the appropriate scale when reading the angle. For example, the angle below is 50° and not 130°:

The math videos below will explain more on this.

Math Video

Video on parts of a protractor & measuring angles that are <180°

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Video on measuring angles that are >180° using a protractor

Math Video Transcript

USING A PROTACTOR (MEASURING ANGLES)

00:00:04.070
In this lesson, we will learn how to use a protractor to measure angles.

00:00:10.220
Now, this is a protractor. The point here is the 'origin'. This is the base line, the inner scale the outer scale.

00:00:33.060
Knowing this, let's use this protractor to measure angles.

00:00:38.070
Measure this angle, ABC. First, we place the protractor's origin, on the vertex B.

00:00:48.150
Next, make sure that the base line is along the line BC.

00:00:54.120
Looking at the inner scale, we can see that this angle is 70 degrees.

00:01:01.000
Hence, the angle ABC is equals to 70 degrees.

00:01:08.170
Next example, find the value of this angle, ABC.

00:01:14.130
To do so, we place the protractor's origin, on the vertex B.

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Next, make sure that the base line is along the line BC.

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Now, from the inner scale, the angle is measured as, 51, 52, 53, 54, 55 degrees.

00:01:32.140
Hence, the angle ABC is equals to, 55 degrees.

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Next example, measure the angle, ABC.

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Similarly, we place the protractor's origin on the vertex B, and make sure the base line is along the line, BC.

00:01:55.110
From the inner scale, we can see that, this angle is 130, 131, 132 degrees.

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Hence, the angle ABC is equals to 132 degrees.

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That is all for now.


--Using a Protractor to measure angles greater than 180 degrees--

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In this lesson, we will learn on using a protractor to measure any angle that is greater than 180 degrees.

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Now, measure this angle, ABC.

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To measure this angle, we split this angle into 2 parts, by drawing a dotted line along BC. 

00:00:24.050
By finding the sum of these 2 angles, we will get the angle ABC.

00:00:29.120
Now, we place the protractor, at the vertex B. 

00:00:34.170
Next, we rotate the protractor to that its base line is along BC.

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Here, we can see that, this angle is 180 degrees.

00:00:50.050
Next, to measure this angle, we rotate the protractor, so that its baseline is along the line AB.

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From the outer scale, we can see that, this angle is 40 degrees.

00:01:08.080
Hence, the angle ABC is equals to 180 degrees, plus 40 degrees.

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This gives 220 degrees. Therefore, the angle ABC is 220 degrees.

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Next example, measure this angle x using a protractor

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By now, we should know that the angle for a full rotation is 360 degrees.

00:01:38.150
Therefore, we can find x, by taking 360 degrees, minus this angle.

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So, let’s measure this angle using a protractor.

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To do so, we place the protractor's origin, here.

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Then, make sure that the base line is along this line.

00:02:03.060
From the outer scale, we can see that this angle is 60, 61, 62, 63, 64, 65 degrees.

00:02:24.000
Hence, x is equals to, 360 degrees, minus by 65 degrees. This gives 295 degrees.

00:02:36.080
That is all for this lesson, try out the practice question to further your understanding.

Practice Questions & More

Multiple Choice Questions (MCQ)

Now, let's try some MCQ questions to understand this lesson better.

You can start by going through the series of questions on using a protractor or pick your choice of question below.

Question 1 on measuring an angle that is lesser than 180°
Question 2 on measuring an angle that is greater than 180°

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