- 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.

## Things You Will Need

- Measuring tape
- Calculator
- Pen and paper