Enter your roof footprint and pitch to find the true roof area in squares, the number of shingle bundles you need, and estimated cost — with a waste allowance.
You can't just use your house's footprint — a sloped roof has more surface than the ground it covers. The trick is the pitch factor (or roof multiplier): measure the footprint, multiply by a factor based on the slope, and you get the true roof area. Then convert to squares (100 sq ft each), the unit roofers actually buy and price by.
1. Footprint area = length × width (include overhangs).
2. Roof area = footprint × pitch factor, where pitch factor = √(rise² + 12²) ÷ 12.
3. With waste = roof area × (1 + waste % ÷ 100).
4. Squares = area ÷ 100. Bundles = squares × 3 (asphalt shingles).
| Pitch | Multiplier | Slope |
|---|---|---|
| 3 in 12 | ×1.031 | Low slope |
| 4 in 12 | ×1.054 | Common, walkable |
| 6 in 12 | ×1.118 | Standard pitch |
| 8 in 12 | ×1.202 | Steep |
| 12 in 12 | ×1.414 | Very steep (45°) |
Add about 10% for a simple gable roof and 15% or more for roofs with several hips, valleys, dormers, or skylights — each of those means more cuts and offcuts. You also need extra shingles for starter rows and ridge caps, which this allowance helps cover. When in doubt, round bundles up.
This gives a solid material estimate for a straightforward roof, but complex rooflines are hard to measure from the ground. For a real quote — and before climbing up there — have a roofer confirm the measurements and condition of the decking underneath.