Prisms are polyhendric geometric forms that come in several forms --- that means that they are solid and exist in three dimensions. It also means that their surface area is flat --- unlike a sphere --- and that they are marked by straight edges, like a square or rectangle. Finding the perimeter of a prism is fairly easy to do and requires only several basic calculations.
Find a prism. If using an actual prism, place the object on a flat surface. Or, if using an image of a prism, determine whether the image represents the true proportion of the object or a ratio.
Measure the length of each of the sides of the base. Types of prisms vary --- hexagonal, triangular, pentagonal, for instance. So, with some prisms you will have more measurements to make than with others.
Add the measurements of each side of the base. The resulting figure is the total length of the perimeter of the prism. For instance, if the three sides of a triangular prism measure one, two and three inches, the perimeter of the prism will be six inches.