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.
Roofing is counted in squares, where one square covers 100 square feet. The catch is that the roof area is larger than the building footprint because the roof slopes — multiply the footprint by the pitch (slope) factor to get the true surface area before dividing by 100. A 4/12 roof is about 5% larger than its footprint; a 12/12 roof is over 40% larger.
Shingles are only the field material. Add underlayment (felt or synthetic) over the whole deck, starter strip along the eaves, ridge cap along the hips and ridges, and drip edge around the perimeter — each estimated by area or linear foot. Valleys, flashing, and penetrations (vents, chimneys) are separate line items that the bare square count misses.
100 square feet of roof surface — the standard unit for ordering shingles.
A roofing square is 100 square feet of roof surface. Roofing materials and labor are quoted in squares, so the estimate converts total area to squares and rounds up.
Multiply the building footprint by the pitch factor for your slope. A 4/12 roof's surface is about 5% larger than the footprint, while a 12/12 roof is more than 40% larger.
Plan underlayment over the whole deck, starter strip at the eaves, ridge cap along ridges and hips, drip edge around the perimeter, plus valley flashing and penetration flashing as separate items.