The continuous closed line that makes the boundary of a two-dimensional shape is known as the perimeter. It’s easy to work out the perimeter of a shape with straight edges, such as a square, because you simply add the lengths of each edge together. A different formula is required to work out the perimeter of a semi circle (a circle cut in half), because it consists of a curved edge as well as a straight edge. That formula is P = 1/2 (π × d) + d, where d is the diameter of the semi circle.

## Remember Pi Constant

Apply the number pi (π) to the formula P = 1/2 (π × d) + d. Pi is a circle's circumference divided by its diameter and is always the same value, 3.14.

## Note the Diameter

To find the perimeter of a semi circle, you have to know the diameter (the length of its straight edge). For example, if the diameter of your semi circle is 12 centimeters, the formula becomes P = 1/2 (3.14 × 12) + 12.

## Break Down the Equation

Work out 3.14 x 12 = 37.68. Then work out 37.68 ÷ 2 = 18.84. You now have the perimeter of the curved edge of the semi circle. But a semi circle also has a straight edge, which is why you have to add the length of the diameter to calculate the whole perimeter. In this case, work out 18.84 + 12 = 30.84. The perimeter of your semi circle is 30.84 centimeters.