Roofing Calculator
Enter your roof's footprint dimensions and pitch to calculate the actual sloped roof area in squares (100 sq ft units) and the number of shingle bundles needed.
How the Roofing formula works
Roof pitch increases actual surface area beyond the flat footprint:
Footprint area = Length × Width Sloped area = Footprint area × Pitch multiplier Squares = Sloped area / 100 Bundles = Squares × 3 (standard 3 bundles per square)
Pitch multipliers scale with steepness — a 6/12 pitch (moderate slope) uses roughly 1.118, while a steeper 12/12 pitch uses roughly 1.414.
Step-by-step calculation
- Multiply the roof's footprint length by width to get the flat plan area.
- Multiply by the pitch multiplier for your roof's slope to get the actual sloped surface area.
- Divide by 100 to convert to roofing squares, the standard unit roofers use.
- Multiply squares by 3 for a standard three-bundle-per-square shingle count, then add a waste allowance.
Worked example
A roof with a 40 ft × 30 ft footprint (1,200 ft²) and a 6/12 pitch: Sloped area = 1,200 × 1.118 ≈ 1,342 ft². Squares = 1,342 / 100 ≈ 13.4, so plan for 14 squares and about 42 bundles, plus waste.
Frequently asked questions
What does a '6/12 pitch' mean?
It means the roof rises 6 inches vertically for every 12 inches (1 foot) of horizontal run — a common way roofers express slope steepness, distinct from a plain angle in degrees.
How much waste allowance should I add?
10% is typical for a simple roof shape; roofs with lots of hips, valleys, or dormers often need 15% or more to account for extra cuts.