An octagon is an eight-sided figure, two-dimensional figure. You might need to find the area of an octagon is you were making a stop sign or if you were building an octagonal ring for an Ultimate Fighting Championship-style event. To calculate the area of an octagon, you need to know the length of the sides or the length of the radius of the octagon.

Measure the length of side of the octagon with the ruler.

Square the length of the side. For example, if your sides are each 10 inches long, you would get 100.

Multiply the result from Step 2 by 4.84 to calculate the area of the octagon. For example, you would multiply 100 by 4.84 to find that the area equals 484 square inches.

Measure the radius with your ruler. The radius is the distance from the centre of the octagon to the point where any two sides intersect.

Square the radius from Step 1. For example, if the radius equals 8 inches, you would get 64.

Multiply the result from Step 2 by 2.828 to calculate the area. For example, you would multiply 64 by 2.828 to find that the area of the octagon equals about 180.992 square inches.