To find the surface area of a rectangular prism, find its length, width and height. Multiply width times height, length times width, and length times height. Add the three products together, and multiply the sum by two. The result is the area surface.

**Find the length, width and height**Measure the length, width and height of the rectangular prism. Example measurements for a rectangular prism are 5 inches for the length, 2 inches for the width, and 3 inches for the height.

**Find the area of the base**Multiply the width times the length to find the area of the base. The area of the example's base is 10 inches squared.

**Find the area of a side**Multiply the length times the height to find the area of a side. The example has a side with an area of 15 inches squared.

**Find the area of the adjacent side**Multiply the width times the height to find the area of the adjacent side. The example has an adjacent side of 6 inches squared.

**Find the sum of the products**Add up the products to find the sum. The equation for the sample products is 15 + 10 + 6, which equals 31 inches squared.

**Multiple the sum by two**Rectangular prisms have opposite sides that are equal, so multiply the sum of the products by two to account for the remaining sides. The equation for the example is 31 x 2, which equals 62 inches squared for the total area of the rectangular prism.