How to Calculate the Perimeter of a Shape

Perimeter of Shapes
••• coloured shapes image by Leslie Batchelder from

The perimeter of shapes is the summation of the length of each side of a shape. Perimeter for a circle is different: when a diameter equals one, then the perimeter equals pi. Contractors use perimeter for things such as to determine lengths of fences or putting a border around a room.

Non-Circular Shapes

    Measure the length of each side of a shape. For example, a parallelogram may have sides of 3 inches, 3 inches, 5 inches and 5 inches.

    Add the sides together. In the example, 3 plus 3 plus 5 plus 5 equals a perimeter of 16 inches.

    Recalculate the perimeter to check your work. Since perimeter calculations involve adding three or more sides, each additional side adds to the chance of making a calculation error.


    Measure the diameter of the circle. The diameter is the length from one end of a circle to the opposite end of the circle. For example, a circle may have a diameter of 10 inches.

    Divide the diameter into two to determine the radius. In the example, the radius of the circle is 5 inches.

    Multiply 2 by the radium and pi. In the example, 2 times 5 times 3.14, which equals a perimeter of 31.4 inches.

    Recalculate the perimeter to check your work and eliminate the chances of a mathematical error.

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