Write down the measurements of all your construction materials. Organize your list by type of construction material if you need more than one type, such as carpet, siding or sheet rock.
Next to the name of the material, write the measurement for length and width or length and height. For example, if you need to buy siding, write down the length and height of the siding in feet.
Calculate any square footage for your materials by multiplying the length and width or height and length. For example, if your siding measures 20 feet in length with a height of 8 feet, you need 160 square feet of siding.
Calculate any subtractions from your square footage, such as a garage door. In the example, if one siding is 160 square feet, but you have a garage door that is 16 feet in length and 7 feet in height, you subtract the square footage of the garage door from the square footage of the total siding.
The square footage of the garage door is 112. Subtract 112 from 160, and you get 48 square feet.
Add the total amount of materials in square footage. For total siding, add up the square footage for each wall and garage door siding.
In total, you need siding for three walls measuring 160 square feet and one wall of siding for 48 square feet. Once added, you need 528 square feet of siding.