How Many Shingles Do I Need? A Step-by-Step Formula
The exact formula for calculating how many shingle bundles a roof needs, from footprint and pitch to squares to bundles, with a worked example.
Figuring out how many shingles to buy comes down to three steps: finding your roof's true surface area, converting that to "squares," then converting squares to bundles.
The formula
- Find your building's footprint (length × width), add your eave overhang on all sides.
- Multiply by a slope factor based on your roof pitch: √(rise² + run²) ÷ run.
- Divide the result by 100 to get squares (1 square = 100 sq ft of roof).
- Add a waste factor (typically 10-15%).
- Multiply by 3 (bundles per square for most asphalt shingles) and round up.
Worked example
A 40 ft × 30 ft building, 6/12 pitch, 12-inch overhang: footprint with overhang is 42 ft × 32 ft = 1,344 sq ft. Slope factor for 6/12 is √(6² + 12²) ÷ 12 ≈ 1.118, giving a roof area of about 1,503 sq ft, or 15.03 squares. Add 10% waste: 16.53 squares. Multiply by 3 bundles per square and round up: 50 bundles.
Run your own building's numbers through the Roofing Calculator instead of doing this by hand — it also adds underlayment rolls, nail weight, and an optional cost estimate.