The amount of shingles you need completely depends on the size of your home. To calculate the area of a rectangular or square wall, multiply the width times the length. To find the area of a triangular wall, multiply the height by the width, and divide the sum by two. Once you have the total area of your home’s exterior walls, you can determine how many HardieShingles you’ll need based on the size of the shingles you decide to install.