Measure the length and the width of the rectangle and identify which dimension also serves as the diameter of the half-circle.
Double the length of the side of the rectangle that does not form the diameter of the half-circle. For example, if the side length is 8 inches, multiply 8 by 2 to get 16 inches.
Add the length of the diameter that you found in Step 1. The diameter accounts for only one side of the figure because the other side is where the half-circle meets the rectangle. In this example, if the diameter equals 10 inches, add 10 to 16 to get 26 inches.
Multiply the diameter by 1.57 (or pi divided by 2) to find the perimeter of the arc of the half-circle. In this example, multiply 10 by 1.57 to get 15.7 inches.
Add the perimeter of the three sides of the rectangle plus the perimeter of the arc of the half-circle. In this example, add 26 inches to 15.7 inches to arrive at a perimeter of 41.7 inches.