Measure the length and width in inches of the wall or walls on which the board will be placed.
Multiply these two numbers together to get the total square inches for each wall. For instance, a wall 100 inches by 100 inches would be 100 x 100 = 10,000 inches square.
Note any large openings on that wall such as windows or doors. Measure the length and width of each opening.
Multiply the length and width of each opening together to get the the square inches of these open spaces. For instance, a wall with a 50 inch by 20 inch window would be 50 x 20 = 1,000 square inches.
Subtract the total square inches of openings from the total area of the wall. In the example: 10,000 square inches -- 1,000 square inches = 9,000 square inches.
Multiply the total board area from Step 5 by 15 percent to take into account an inherent percentage of waste from installation. 9,000 square inches x 15 percent (or .15) = 1,350 square inches.
Add the number from Step 6 to the number from Step 5. Continuing with the example, 9,000 square inches + 1,350 square inches = 10,350 square inches.
Multiply the length and width of the sheathing boards you will purchase to get the area of each board. For instance a 2-by-8 inch board would be: 2 x 8 = 16 square inches.
Divide the wall's total area by the area of a single board to determine the number of boards needed. Round up to the nearest number if you get a decimal. In the example: 10,350/16 = 646.875, so you will need 647 2-by-8 inch boards.
Find the square footage of the insulation board by dividing the area of the wall by 12. In the example: 10,350/12 = 862.5 feet square.