HOW MANY SHINGLES YOU WILL NEED
The quantity of materials needed for a job will be based on the surface area of the roof that is to be covered. There is a simple formula for figuring this out:
Length X Width = Area
Make sure you measure actual surface areas and not just the perimeter of your roof. When estimating, factor in cutting, slop, ridge lines, and extra for a dog house.
Three bundles of shingles make a "square". A square theoretically covers 100 square feet. I have found this rule of thumb to be accurate if you don't make too many mistakes with the utility knife.