A semicircle is half of a circle. It is a two-dimensional figure obtained when a circle is cut into two halves. Its shape consists of an arc and a segment.

The radius of a semicircle is defined as the distance from the midpoint of the segment to any point of the arc.

The perimeter of a semicircle with radius $r$ is the length of the segment plus half the circumference of a circle with radius $r.$ The area of a semicircle with radius $r$ is half the area of a circle with radius $r.$

Perimeter	Area
$P=2r+\pi r$	$A=\frac{1}{2}\pi r^2$