Definition, Formula, Examples | Octagon Shape - Cuemath

Octagon

Octagon is an eight-sided two-dimensional geometrical figure. An octagon consists of 8 interior angles and 8 exterior angles. The sum of the interior angles of an octagon is 1080°, and the sum of its exterior angles is 360°. There are 20 diagonals in an octagon. Octagons are classified into various types based upon their sides and angles. Let us learn more about the octagon shape in this article.

1.	What is an Octagon?
2.	Types of Octagons
3.	Properties of an Octagon
4.	FAQs on Octagon

What is an Octagon?

An octagon can be defined as a polygon with eight sides, eight interior angles, and eight vertices. When all the sides and angles of an octagon are equal in measurement, it is called a regular octagon. Every polygon is either convex or concave. Convex octagons bulge outwards, whereas concave octagons have indentations (a deep recess). Convex octagons are those in which all the angles point outwards. A regular octagon is an example of a convex octagon. The octagon in which at least one of its angles points inwards is a concave octagon. Observe the figure given below to see what an octagon looks like.

Octagon Sides

An octagon is a polygon with 8 sides and 8 interior angles. The word 'Octagon' is derived from the Greek word, 'oktágōnon' which means eight angles. That is the reason why it is called an octagon.

Types of Octagons

Depending upon the sides and angles, an octagon is classified into the following categories:

• Regular and Irregular Octagon
• Concave and Convex Octagon

Regular Octagon

The octagon that has eight equal sides and eight equal angles is known as a regular octagon.

• In a regular octagon, all the sides are equal in length, and all the angles are equal in measure.
• The interior angles add up to 1080° and the exterior angles add up to 360°.
• The interior angle at each vertex of a regular octagon is 135°.

Irregular Octagon

An octagon in which the sides and angles are not congruent is an irregular octagon. In other words, an irregular Octagon has eight unequal sides and eight unequal angles.

• It is an octagon with unequal sides and angles.
• All the interior angles are of different measure, but their sum is always 1080º.

Convex Octagon

The octagon in which each interior angle is less than 180° is a convex octagon.

• Convex octagons bulge outwards.
• None of their interior angles is greater than 180°.

Concave Octagon

The octagon in which one of the angles points inwards is a concave octagon.

• Concave octagons have indentations (a deep recess).
• The interior angles are greater than 180°, that is, at least one angle is a reflex angle.

Properties of an Octagon

Here are a few properties of an octagon that can help to identify it easily.

• An octagon is a polygon with eight sides and eight angles.
• All its interior angles sum up to 1080°
• 6 triangles can be formed in a regular octagon with the help of diagonals using a common vertex.
• It has 20 diagonals.

Octagon Diagonals

The diagonal of an octagon is the line segment that connects any two non-adjacent vertices. There are 20 diagonals in an octagon. The formula that is used to find the number of diagonals in any polygon is, Number of diagonals =n(n-3)/2; where 'n' represents the number of sides of the polygon. In this case, there are 8 sides in an octagon. After substituting the value of 'n' = 8 in the formula, we get, Number of diagonals =n(n-3)/2 = 8(8 - 3)/2 = (8 × 5)/2 = 20. Therefore, there are 20 diagonals in an octagon.

Angles of an Octagon

There are 8 interior angles and 8 exterior angles in an octagon. In a regular octagon, each interior angle is 135°. The sum of an octagon's interior angles is 1080°, and the sum of the exterior angles of an octagon is 360°.

• The sum of the interior angles of an octagon can be calculated with the help of the following formula where 'n' represents the number of sides (8) in an octagon. Sum of interior angles of a polygon = (n - 2) × 180° = (8 - 2) × 180° = 1080°.
• The sum of the exterior angles of an octagon is 360°.

Area of Octagon

The area of an octagon is the total space occupied by it. The formula to calculate the area of a regular octagon is, Area of a Regular Octagon = 2a2(1 + √2); where 'a' is any one side length of the octagon. It is expressed in square units like inches2, cm2, and so on. For example, if one side of a regular octagon is 6 units, let us find the area of the octagon. Area of octagon = 2a2(1 + √2), Substituting the value of 'a' = 6, Area of octagon = 2 × (62) × (1 + √2) = 72 × (1 + √2) = 173.8 square units.

In case of an irregular octagon, there is no specific formula to find its area. We divide the octagon into smaller figures like triangles. Then, after calculating the area of all the triangles, we add their areas to get the area of the octagon.

Perimeter of an Octagon

The perimeter of a polygon is the total length of its boundary. In order to calculate the perimeter of an octagon, the length of all the sides should be known. We know that in a regular octagon, all the sides are of equal length. Therefore, the formula that is used to find its perimeter is,

Perimeter of an octagon = Sum of all its sides

Perimeter of a regular octagon = 8a (Where 'a' is the length of one side of the octagon)

Important Notes

• An octagon has eight sides.
• The sum of all the interior angles in an octagon is always 1080º.
• The sum of all the exterior angles in an octagon is always 360º.
• A regular octagon has 20 diagonals.
• Regular octagons are always convex octagons, while irregular octagons can either be concave or convex.

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