Multiply the length of each side of the triangle by the depth of the prism separately, then multiply the base of the triangle by its height. Add these four figures together.
For right triangles, use the length of the two sides (not the hypotenuse) for the base and height. For a right triangular prism with sides a and b, hypotenuse c and prism depth d, the formula is: area = ad + bd + cd + ab. For non-right triangles, use Heron's formula to determine the area of the triangle and substitute that figure multiplied by two for ab. Find the volume of a triangular prism by multiplying ab/2 by d.