How it's calculated
bundles = ⌈ (sloped area ÷ 100) × bundles per square ⌉, sloped area = footprint × √(1 + (rise/12)²)
footprint = length × width, rise/12 = roof pitch, √(1 + (rise/12)²) = pitch (slope) factor. 1 square = 100 ft²; bundles per square ≈ 3 (3-tab) or 2.2–2.5 (architectural). Waste % is added before rounding up.
Common questions
How many bundles are in a roofing square?
For standard 3-tab and architectural shingles, three bundles cover one square, which is 100 square feet of roof.
Why add the pitch?
A steeper roof has more sloped surface than its flat footprint. The pitch multiplier converts ground area into the real area you must cover.