Roofing Calculator
Work out exactly how many shingle bundles, underlayment rolls and ridge cap you need, based on your roof's footprint and pitch. Add your own prices for a full cost estimate.
How this calculator works
Pitch converts your roof's flat footprint into actual sloped surface area, since a steeper roof has more material to cover for the same building size underneath:
roof area = footprint area × pitch multiplier squares = roof area ÷ 100 bundles = ceil( squares × bundles per square × (1 + waste%) )
The pitch multiplier comes from the roof's rise over a 12 inch run: a 6/12 pitch rises 6 inches for every 12 inches of horizontal run, which works out to a multiplier of about 1.118 using √(rise² + 12²) ÷ 12.
Worked example
A 40 ft × 30 ft footprint, 6/12 pitch, architectural shingles, 10% waste:
- Footprint: 40 × 30 = 1,200 sq ft.
- Roof area: 1,200 × 1.118 ≈ 1,342 sq ft.
- Squares: 1,342 ÷ 100 ≈ 13.4 squares.
- Bundles: 13.4 × 3.33 × 1.10 ≈ 49.1 → buy 50 bundles.
Pitch multiplier reference
| Pitch | Multiplier | Extra area vs footprint |
|---|---|---|
| 4/12 | 1.054 | +5.4% |
| 6/12 | 1.118 | +11.8% |
| 8/12 | 1.202 | +20.2% |
| 10/12 | 1.302 | +30.2% |
| 12/12 | 1.414 | +41.4% |
If your roof falls between these — say a 5/12 or 7/12 — use the "Custom rise" option above rather than rounding to the nearest standard pitch; the multiplier is calculated directly from your exact rise.
Waste, valleys and complex rooflines
A simple gable roof with no valleys can often get away with close to the standard 10% waste allowance. A roof with several hips, valleys, dormers or a cut-up roofline creates more angled cuts and offcuts that can't be reused elsewhere, so budgeting 12–15% waste is safer for anything beyond a straightforward rectangular roof.
Frequently asked questions
What is a roofing square?
A roofing square is 100 square feet of actual roof surface area, measured along the slope rather than the flat footprint below it. Roofers and suppliers price and order materials by the square, which is why this calculator converts footprint into squares before working out bundles.
How does roof pitch change the amount of material needed?
A steeper pitch means more actual surface area for the same building footprint, since the slope adds length a flat measurement doesn't capture. A 12/12 pitch has about 41% more surface area than its footprint, while a gentle 4/12 pitch adds only about 5%.
How many bundles per square for architectural shingles?
Standard 3-tab shingles typically use 3 bundles per square. Architectural, or dimensional, shingles are thicker and often run 3 to 4 bundles per square depending on the manufacturer, so check your specific product's coverage rating rather than assuming a fixed number.
Do I need extra shingles for a roof with lots of valleys?
Yes. Hips, valleys and a complex roofline all create extra cuts and waste beyond a simple gable roof, so a waste allowance toward the higher end of the typical 10 to 15 percent range is worth budgeting for a roof with several intersecting planes.