Pitch makes the real (sloped) roof bigger than its flat footprint. We apply the standard slope factor, surface it in the result so you can trust the correction, convert the corrected area to roofing squares (1 square = 100 ft²), add your waste allowance, and round up to whole bundles only at the very last step.

multiplier   = √(rise² + 12²) / 12        (rise:12 pitch; 0/12 = 1.0)
roof_area    = footprint_ft² × multiplier
squares      = roof_area / 100            (kept as a decimal)
squares_adj  = squares × (1 + waste%)
bundles      = ceil(squares_adj × bundles_per_square)
shingles     = bundles × 29
companion (optional):
  felt_rolls = ceil(roof_area / 400)      (15# felt; 200 for 30#)
  nails      = round(squares × 320)       (3-tab)
cost (optional):
  material   = squares_adj × price_per_square

Worked example — 50 ft × 30 ft footprint (1,500 ft²) at 6/12 with 10% waste and 3 bundles/square: multiplier ×1.1180, roof area ≈ 1,677 ft², ≈ 16.77 squares → ≈ 18.45 with waste → 56 bundles and 1,624 shingles. Bundles come from the un-rounded adjusted squares, not a re-rounded square count — which prevents the common off-by-a-bundle error.

Defaults reflect standard architectural asphalt shingles (3 bundles/square, 29 shingles/bundle) and US 2026 conventions from Omni Calculator and Inch Calculator. This is an estimate and an ordering aid — verify quantities against your roof and supplier before ordering.

Formula: multiplier = √(rise² + 12²) / 12 · squares = roof ft² / 100 · bundles = ⌈squares × (1+waste) × bundles/sq⌉

See Sources below