Finding Perimeter and Circumference

Updated May 8, 2019 | Infoplease Staff

Perimeter

The perimeter of a geometrical figure is the complete distance around that figure. To find the perimeter, simply add up the lengths of the figure's sides.

Example: What is the perimeter of a rectangle that measures 3 inches by 8 inches?

• Since a rectangle has four sides, and its parallel sides are equal in length, this rectangle must have two sides that are each 3 inches in length, and two sides that are each 8 inches in length.
• The sum of the lengths is 3 + 3 + 8 + 8 = 22. Therefore, the perimeter of the rectangle is 22 inches.

Example: What is the perimeter of a pentagon in which each side measures 4 inches?

• A pentagon is a figure with five sides, and this pentagon has all sides of the same length.
• The sum of the lengths is 4 + 4 + 4 + 4 + 4 = 20. Therefore, the perimeter of the pentagon is 20 inches.

Example: What is the perimeter of a triangle that has sides measuring 12 inches, 9 inches, and 6 inches?

• The perimeter is the sum of the lengths of the triangle's sides.
• The sum of the lengths is 12 + 9 + 6 = 27. Therefore, the perimeter of the triangle is 27 inches.

Circumference

The circumference of a circle—the complete distance around a circle—can be found by using this formula:

C = π2r, in which π is about 3.1416 and r is the radius.