Measure the length and width of all the surfaces on your roof in feet. Write down each length and width on a piece of paper.
Multiply each length and width in any order you want. Write the surface area in square feet next to each item on your list. Add all the areas together to get the total roofing surface area.
Divide the total area by 100 and multiply it by 3. Buy this number of bundles of shingles. Be sure to add about 10 percent extra to the bundle number to account for breakage and other accidents.