Question: how to figure out the circumference of a half circle?
Answer: Perimeter of a Semicircle The formula for a circle's circumference, C, is C = 2 * pi * r, where r is, again, the circle's radius. A semicircle is half a circle, however, so we'll need to divide the formula for perimeter by two just like we did the formula for area. The perimeter P = pi * r.