Convert your building's footprint into actual roof surface area, accounting for pitch, then get the number of shingle squares and bundles to order.
A roofing square equals 100 square feet of roof surface — it's the standard unit contractors use to order shingles and materials.
A steeper pitch increases the actual roof surface area compared to the building's footprint, so steeper roofs need proportionally more shingles, underlayment, and materials.
Most standard 3-tab and architectural shingles use 3 bundles per square, though some premium or heavier shingles may use 4–5 bundles per square — check your product's packaging.