A rectangular prism consists of three different dimensions. The prism's length, height and width create its volume and surface area, which are respectively its internal and external measurements. When two of the dimensions and either the volume or surface area are known, the third dimension can be found. You can find the width of a rectangular prism through the formulas for volume and surface area, which are volume = length x height x width, and surface area = 2 x length + 2 x height + 2 x width.

### With Volume

Obtain the length, height and volume of a rectangular prism. In this example, let the length be 20 inches, the height be 20 inches and the volume be 4,000 cubic inches.

Multiply the length with the height. In this example, multiplying 20 inches by 20 inches results in 400 square inches.

Divide the volume by the product of the length and height to calculate the rectangular prism's width. Concluding this example, 4,000 cubic inches divided by 400 square inches equals 10 inches. The rectangular prism's width is 10 inches.

### With Surface Area

Obtain the length, height and surface area of a rectangular prism. In this example, let the length be 10 inches, the height be 10 inches and the surface area be 400 square inches.

Multiply the length by the height, and then double the product. In this example, multiplying 10 by 10 results in 100 square inches, which multiplied by 2 equals 200 square inches.

Subtract the doubled product of the height and length from the surface area. In this example, subtracting 200 square inches from 400 square inches results in 200 square inches.

Double the length, then double the height, and then add the two products together. In this example, 10 inches multiplied by 2 equals 20 inches, so both the length and height doubled equals 20 inches. Adding 20 inches to 20 inches equals 40 inches.

Divide the difference from Step 3 by the sum from Step 4. Concluding this example, dividing 200 square inches by 40 inches equals 5 inches. The width of the rectangular prism is 5 inches.