Squares, bundles and roof area
Roofing is estimated in squares, where one square equals 100 square feet. The first step is finding the true sloped area of the roof, which is larger than the footprint because the surface rises and falls with the pitch. Dividing that area by 100 gives squares, and multiplying squares by bundles-per-square gives the bundle count.
Planning a shingle order
Add a waste allowance suited to the roof's complexity, then account for starter strips and ridge caps, which are not part of the field area. Ordering all shingles from the same batch keeps the color consistent, and keeping a few spares helps with future repairs after storms or damage.