Measure the area of your roof. Divide that by 100. That will give you the number of 'squares' you need. A square is ten square feet and equals three bundles of shingles. So if your roof is 450 square feet, you'd need fourteen bundles. You may want to buy an extra bundle for ridges and valleys. It's easier and more convenient to buy extra and return your unused bundles than to run out in the middle of a job.