Measure the distance from the center of the circle of which the semicircle is part to its edge. This is its radius.
Multiply the radius of the semicircle by itself.
Find the product of pi, which is rounded to 3.14, and the square of the radius.
Using the formula for finding the area of a semicircle, divide the product of the squared radius and pi by 2. If the radius of the semicircle is 2 centimeters, then the area is 1/2(3.14 x 2 x 2), or 6.28 square centimeters.