Examine the object at hand to determine the type of prism it is. You should also check the congruent side and determine its shape.
Place the ruler on one vertex of the object and measure along that edge. Ensure a perpendicular eye level to the ruler so as to avoid parallax errors. Record the observed measurement.
Measure another vertex along the edge just as you did in the previous step. Record the measurement. Continue measuring and recording the edges until you measure them all.
Multiply the length by width of the rectangular faces to find the area. Because the rectangular faces have different dimensions, apply the formula independently for each face.
Calculate the area of one of the congruent side. If the side is a square or rectangle, multiply the length by width; if the side is a triangle, multiply half the length of the base by the height. Because the faces are congruent, multiply by 2 to find the combined area of both.
Add the areas of all rectangles together. Add the sum of all the areas to the combined area of the two congruent sides to find the total surface area of the prism.