Many three-dimensional objects have two-dimensional shapes as parts or components. A rectangular prism is a three-dimensional solid with two identical and parallel rectangular bases. The four sides between the two bases are also rectangles, with each rectangle being identical to the one across from it. The rectangular prism's surface area combines the areas of all six rectangles, which you can find through its three dimensions of height, length and width.
Measure the rectangular prism’s length, width and height. For example, 8 inches long, 6 inches wide, 10 inches high.
Multiply the length by the height, then double the product. With this example, 8 inches multiplied by 10 inches results in 80 square inches, which multiplied by 2 equals 160 square inches.
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Multiply the width by the length, then double the product. With this example, 6 inches multiplied by 8 inches equals 48 square inches, which multiplied by 2 equals 96 square inches.
Multiply the height by the width, then double the product. With this example, 10 inches multiplied by 6 inches results in 60 square inches, which multiplied by 2 equals 120 square inches.
Sum the amounts from Steps 1 through 3 to find the rectangular prism's surface area. So adding 160, 96 and 120 square inches results in 376 square inches.