What this calculator does
The roof shingle calculator estimates how many roofing squares (100 sq ft units) and shingle bundles you need, accounting for roof pitch and a waste allowance for cuts and overlaps.
The formula
Because a sloped roof has more surface area than its flat footprint, the calculator first applies a pitch multiplier: multiplier = √(144 + pitch²) ÷ 12, where pitch is the rise per 12 inches of run (a standard way roof slopes are expressed, e.g. a “6/12” pitch). Multiplying the flat footprint area by this factor gives the actual sloped roof area. A waste percentage (commonly 10%, higher for complex roofs with many valleys and hips) is then added, and the total is divided by 100 to get roofing squares. Bundles needed is squares × bundles-per-square, rounded up, since shingles are typically packaged three bundles per square (though some products vary).
- Complex roofs with multiple valleys, hips, dormers, or a steep pitch may need a higher waste allowance.
- Always add extra for ridge cap shingles and starter strips, which are ordered separately from field shingles.
This tool is for general estimating and project-planning purposes.