How it works
A sloped roof has more surface than the footprint below it. Multiplying the footprint by the slope factor, the hypotenuse of the pitch triangle for one foot of run, gives the true surface area. For any roof with one pitch this is right even for hip roofs.
A roofing square is 100 square feet of roof. Shingle bundles are packaged so that three bundles cover a square. Add an allowance for cuts and waste, then round up to whole bundles. Ridge caps, starter strips, and underlayment are separate.
Worked example
A 40 ft × 30 ft building with 12 in overhangs and a 6/12 pitch:
- Footprint = (40 + 2 × 1) × (30 + 2 × 1) = 1,344 ft²
- Pitch factor = √(1 + (6 ÷ 12)²) = 1.118
- Roof area = 1,344 × 1.118 = 1,503 ft²
- Squares = 1,503 ÷ 100 × 1.1 = 16.5
- Bundles = 16.5 × 3, rounded up = 50
| Input | Value |
|---|---|
| Building length | 40 ft |
| Building width | 30 ft |
| Overhang on each side | 12 in |
| Pitch: rise per 12 in of run | 6 in per 12 |
| Waste allowance | 10 % |
| Result | Value |
|---|---|
| Bundles of shingles | 50 bundles |
| Squares with waste | 16.5 squares |
| Roof surface area | 1,503 ft² |
| Pitch factor | 1.118 × |
| Footprint including overhangs | 1,344 ft² |
Assumptions and limits
- One pitch over the whole roof, with gable or hip ends measured by the footprint including overhangs. Dormers, valleys, and separate pitches need to be added separately.
- Three bundles per square is typical for both 3-tab and architectural shingles; check the package, since some products differ.
- Ridge cap, starter, drip edge, and underlayment quantities are not included.
Common questions
How many bundles of shingles are in a square?
Three for most 3-tab and architectural shingles; each bundle covers about 33 square feet.
How do I account for roof pitch?
Multiply the flat footprint by √(1 + (rise ÷ 12)²). A 6/12 roof multiplies by about 1.118; a 12/12 roof by 1.414.
How much waste should I add?
It depends on the roof’s complexity. Simple gables need less; many hips and valleys need more. This tool lets you set it.
Does this include the ridge cap?
No. Order ridge and hip caps, starter strips, and underlayment separately.
Sources
- A roofing square is 100 square feet; shingles are typically sold three bundles per square (Roofr, RoofSnap references).
- Pitch factor = √(1 + (rise ÷ 12)²), the length of the slope for one foot of run.
Updated 2026-09-30