Measure the parallel sides of the trapezoid and the height of the trapezoid. The trapezoid's height is from one of the parallel side to the other parallel side. For example, assume the sides measure 4 inches and 6 inches. The height is 3 inches.
Add the two sides length's together, then divided by two. In the example, 4 plus 6 equals 10, then 10 divided by 2 equals 5 inches. This is the average length of the sides.
Multiply the average length of the sides by the height of the trapezoid to calculate the surface area. In the example, 5 times 3 inches equals 15 inches squared.