Calculate roof area, squares and shingle bundles from footprint and pitch.
A pitched roof covers more area than its footprint because the surface follows the slope. The pitch multiplier √(1 + (rise/12)²) converts horizontal footprint area to actual roof area — it's the ratio of the rafter length to its horizontal projection. A roofing square is 100 sq ft, the unit used for ordering shingles. Bundles per square depend on the shingle type: 3 for standard 3-tab, 4 for architectural/dimensional.
Pitch multiplier
Pitch multiplier = √(1 + (rise/12)²)
Actual roof area
Roof area = footprint area × pitch multiplier × (1 + waste%)
Squares and bundles
Squares = area ÷ 100; Bundles = ceiling(squares × bundles_per_square)
A roofing square is 100 square feet of roof surface. It's the standard ordering unit for shingles. A 2,000 sq ft roof = 20 squares.
Standard 3-tab strip shingles use 3 bundles per square. Architectural (dimensional or laminated) shingles typically require 4 bundles per square because they're heavier. Check your manufacturer's data sheet.
For simple gable and hip roofs it is exact. Complex roofs with dormers, valleys or multiple pitches should be measured by facet — use the 'Number of Facets' field to account for multiple planes.
It's the plan view (bird's-eye) area of the structure the roof covers, not the roof surface itself. If your house is 40 ft × 30 ft, the footprint is 1,200 sq ft. The actual roof area will be larger depending on the pitch.