Roofing is sold by the square (100 square feet), and the trick is converting a flat footprint into true sloped area. Use the roofing calculator to get squares and bundles from your footprint and pitch, the shingle calculator for bundle counts with waste, the roof pitch calculator for pitch, angle, and slope multiplier, and the rafter length calculator for cut lengths. The snow load calculator gives a planning-level roof load estimate.
Read How to Estimate Roofing for the full workflow, or browse the roofing FAQ. Calculations follow ARMA and NRCA conventions (three bundles per square for architectural shingles).