Formula for Volume of a Trapezoidal Prism. If the prism length is L,trapezoid base width B, trapezoid top width A, and trapezoid height H, then the volume of the prism is given by the four-variable formula: V(L, B, A, H) = LH(A + B)/2. In other words, multiply together the length, height, and average of A and B. -wuora