How do you determine the amount of shingles you need to cover a roof?
Measure the roof surface carefully. Each 100 square feet is called a square. There are usually 3 or 4 bundles in a square. Ask your supplier how many bundles in a square of your choice of shingle or give him the square footage and he will tell you.
Also measure the eaves of the roof, the part that has the gutter attached. This gives the linear feet of starter shingle and eave protection, usually of 50 pound, non-perforated roofing felt. Also measure the ridge and hips for shingle caps. Finally measure the valleys for valley flashing. It is a good idea to replace chimney flashings that go on the roof, other flashings and roof vents, as well.