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Angle Measurements


Hello, mathematicians-in-the-making! We will talk about a fascinating part of geometry – Angle Measurements. Have you ever wondered how we determine that a circle has 360 degrees? Or why is the right angle always 90 degrees? These concepts all come from studying angle measurements. So, fasten your seatbelts and prepare to explore the mysterious world of angles!

Grade Appropriateness

This topic is typically introduced to students ages 8 to 12. It is suitable for children in 3rd to 7th grade, depending on their math curriculum and their pace of learning.

Math Domain

Angle Measurements fall under the domain of Geometry in Mathematics. Geometry uses shapes and space to help us understand and explain the world around us.

Applicable Common Core Standards

The concepts related to Angle Measurements address the following Common Core Standards:

4.G.A.1: Draw and identify lines and angles and classify shapes by properties of their lines and angles.

4.MD.C.5a: Understand concepts of angle measurement.

4.MD.C.6: Measure angles in whole-number degrees using a protractor.

Definition of the Topic

An ‘angle’ in geometry is the space or region between two intersecting lines or surfaces at the point where they meet. Angle measurements tell us the size of the angle and are usually measured in degrees (°).

Key Concepts

There are several types of angles based on their measurements:

Acute Angle: This is an angle less than 90°.

Right Angle: This is an angle of exactly 90°, often marked by a small square.

Obtuse Angle: This is an angle between 90° and 180°.

Straight Angle: This is an angle of 180°.

Reflex Angle: This is an angle greater than 180° but less than 360°.

Full Angle: An angle of 360° makes a complete circle.

Discussion with Illustrative Examples

In geometry, an angle is the figure created when two rays intersect at a point known as the vertex. Angle measures the amount of turn of its rays in degrees.

Arms: The two rays joining to form an angle are called arms of an angle. Here, WH and WM are the arms of the ∠HWM.
Vertex: The vertex is the common endpoint of the two rays where they meet. Here, the point W is the vertex of ∠HWM.

Types of Angles Based on Measurements

Acute Angles: These angles are less than 90°
Right Angles: These angles measure exactly 90°
Obtuse Angles: These angles measure more than 90° but less than 180°.
Straight Angles: These angles measure exactly 180°
Reflex Angles: These angles measure more than 180° but less than 360°
Full Rotation Angles: These angles measure exactly 360°

Let’s make this more fun with a few examples:

Acute Angle: Imagine you open a book slightly. The angle between the open pages forms an acute angle.

Right Angle: The corner of a square or rectangle is a right angle.

Obtuse Angle: Picture a ‘V’ shape. The angle inside the ‘V’ is obtuse.

Straight Angle: The angle at which the hands of a clock are at 6 o’clock is straight.

Reflex Angle: If you open a door halfway, the angle between the open door and the door frame is reflex.

Full Angle: The angle covered when you spin around once is a full angle.

Drawing Angles Using a Protractor

A protractor measures angles, typically a flat semicircle marked with degrees along the curved edge.

Begin by using the protractor’s straight edge to draw the first ray.
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.
Use the straight edge to connect the mark with the endpoint of the first ray.
Label the angle with the correct measurement.

Measuring Angles Using a Protractor

Determine the angle’s vertex or center point.

Place the origin/center point of the protractor over the vertex.

Align the protractor’s bottom edge with one of the angle’s rays or edges.

Read the measurement of the angle.

Examples with Solutions

Example 1

If you have an angle of 45 degrees, what type of angle is it?


Since it is less than 90 degrees, a 45-degree angle is an Acute Angle.

Example 2

What type of angle is 180 degrees?


A 180-degree angle is a Straight Angle.

Example 3

Identify the name of the angle, the measurement of the angle, and the classification of the angles.


a. Name of the Angle:  ∠MGB or ∠BGM    
Measure of the Angle: 90°    
Classification of the Angle: Right Angle
b. Name of the Angle:  ∠DEF or ∠FED    
Measure of the Angle: 130°    
Classification of the Angle: Obtuse Angle

Real-life Application with Solution

Suppose your digital clock shows 3:00. What angle is formed between the hour and minute hands?


At 3:00, a clock’s hour and minute hands form like a corner of a square. Therefore, they form a right angle of 90 degrees.

Practice Test

1. Classify the following angles: 90 degrees, 200 degrees, 45 degrees, 180 degrees, 10 degrees.

2. Tommy drew an angle of 150 degrees. What type of angle did Tommy draw?

3. Sara opens her book to an angle of 80 degrees. What type of angle does this create?

4. If an angle measures 360 degrees, what is its classification?


1. 90 degrees –  Right Angle, 

    200 degrees – Reflex Angle

    45 degrees – Acute Angle

    180 degrees – Straight Angle

    10 degrees – Acute Angle

2. Obtuse Angle

3. Straight Angle

4. Full Rotation

Frequently Asked Questions (FAQs)

Why do we measure angles in degrees?

We measure angles in degrees because it is a way to break down a circle into equal parts. The concept comes from ancient Babylonians, who used a counting system based on the number 60.

Can an angle be negative?

Yes, an angle can be negative. Negative angles are measured in the opposite direction of positive angles. Hence, if a positive angle measures the counterclockwise direction, the negative angle measures the clockwise direction.

What is a protractor, and how do I use it?

A protractor is a semi-circle-shaped tool used to measure angles. To use a protractor, you place the center point at the vertex of the angle (where the two lines meet) and align one 

line along the zero of the protractor. The number that the other line crosses is the measure of the angle in degrees.

Is a right angle only 90 degrees?

Yes, a right angle is always 90 degrees. It forms a perfect ‘L’ shape.

What is the smallest possible angle?

The smallest possible angle in a plane is 0 degrees, which occurs when two lines overlap precisely. 

Well, that’s it for our exploration of angle measurements! Remember that understanding and identifying various angles will become easier as you practice. Keep up the excellent effort, and remember to look around you for more real-world instances of angles.

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