Estimate roofing squares and shingle bundles from your roof's footprint and pitch multiplier, with a waste factor matched to roof complexity.
roof_area = footprint × pitch_multiplier → squares = area ÷ 100
Roofers measure in squares — one square is 100 ft² of roof surface. Three bundles of 3-tab shingles typically cover one square.
Use 10% for a simple gable, 12–15% for a hip roof, and 15–20% for a complex multi-valley roof.
A 40 x 28 ft house footprint is 1,120 square feet. For a 6:12 pitch gable roof, the slope multiplier is 1.118, so the actual roof area is 1,120 x 1.118 = 1,252 square feet, or 12.5 squares.
At 3 bundles per square that's 37.5 bundles, and a simple gable carries 10% waste, so about 41 bundles. If this were a hip roof with four sloped planes and diagonal cuts at the corners, you'd use 12-15% waste instead, pushing it to 43-44 bundles. Complex roofs with valleys and dormers go to 15-20%.
Waste factors follow ARMA/NRCA guidance. Get the pitch multiplier from the roof pitch calculator. Architectural shingle bundle counts vary by product.
100 square feet of roof surface — the standard unit for ordering shingles.