The volume of a prism is worked out by first calculating the area of the surface and then multiplying that figure by the length of the prism. This applies whatever the shape a prism takes.
The reason behind this formula are the properties that make up a prism. A prism is a shape that has two identical ends, flat sides, and the same size and shape of cross section throughout its whole length.
Take a triangular prism as an example. The surface area of the end triangle is worked out by the calculation 0.5 multiplied by the base multiplied by the perpendicular height. This can be represented by 1/2 x b x h.
The volume is worked out by taking this figure and multiplying it by the length of the prism. So, for a triangular prism, volume is worked by the calculation: 1/2 x b x h x l.
Another simple example is a square prism, which is also known as a cube. The area of the top side needs to be work out first by the simple calculation of height multiplied by width. This figure should then be multiplied by the length of the prism to get the volume. That makes the calculation for a square prism as follows: volume = h x w x l.