A semicircle measures 180 degrees.
This is because a semicircle is half of a full circle. A full circle encompasses 360 degrees, so when you divide that by two for the semicircle, you get 180 degrees. You can visualize this by thinking of a circle: if you were to draw a diameter, which is a straight line that passes through the center of the circle and touches both sides, the two halves formed are the semicircles. Each of these semicircles will measure 180 degrees.