Roofing Calculator
Squares and bundles of shingles from the building footprint and roof pitch — the standard slope-factor method roofers use.
Result
| Slope factor | |
| Roof area (before waste) | |
| Squares (incl. waste) |
Units & conversions used
Footprint dimensions are in feet (metric inputs convert at 1 m = 3.28084 ft). Shingles are sold by the square: 1 square = 100 ft² of roof area. Most architectural shingles are packed 3 bundles per square — confirm on your product's wrapper, as some heavyweight lines take 4–5.
Formula & source
Roof area = footprint(ft²) × slope factor
Squares = roof area × (1 + waste%) ÷ 100
Bundles = squares × 3
Source: the slope-factor method is the standard estimating approach for pitched roofs — it converts the flat (plan) footprint into true sloped surface area using only the pitch. Common factors:
| Pitch | 4/12 | 6/12 | 8/12 | 12/12 |
|---|---|---|---|---|
| Slope factor | 1.054 | 1.118 | 1.202 | 1.414 |
Three bundles per square is the standard pack for most architectural shingles; the bundle count on your product's wrapper governs.
Worked example (you can verify by hand)
Input: 40 ft × 30 ft footprint, 6/12 pitch, 0% waste (to isolate the slope math).
- Slope factor: √(1 + (6÷12)²) = √1.25 = 1.118
- Roof area: 40 × 30 × 1.118 = 1,341.6 ft²
- Squares: 1,341.6 ÷ 100 = 13.42
- Bundles: 13.42 × 3 = 40.25 → round up
Expected output: slope factor 1.118, roof area 1,341.6 ft², 13.42 squares, 41 bundles.
Limitations — what this cannot do
- This is the flat-projection × slope-factor method. It treats the roof as simple equal slopes; hips, valleys, dormers and multiple roof planes should be measured plane by plane — and waste on such roofs runs 15% or more, not 10%.
- Ridge and hip cap shingles, starter courses, underlayment, drip edge, flashing and vents are not counted. Order cap/starter product separately (often a bundle or two) and roll goods by linear measure.
- Bundles per square varies: most architectural shingles are 3, some heavyweight products 4–5. Verify on the wrapper before ordering.
- Measuring pitch from the ground is error-prone. Measure on the roof or from the attic (level against a rafter over a 12-in run) where it is safe to do so — or leave roof work to a roofer.
FAQ
What is a "square" in roofing?
100 ft² of roof area. A 1,342 ft² roof is a "13.42-square roof" — this is the unit shingles, underlayment and contractor quotes are all priced in.
How much do shingles cost?
Standard architectural shingles commonly sell in the $90–$150 per square range for the material alone (2025–2026), with premium designer lines higher; a full tear-off and replacement commonly costs several times the material figure once labor, underlayment and disposal are in. Get local quotes in squares so you can compare them against your own calculation.
How do I find my roof pitch?
Pitch is rise over a 12-in run. On the roof, set a level on the slope, mark 12 in horizontally, and measure the vertical drop to the roof surface. From inside the attic you can measure against a rafter the same way. Phone pitch-finder apps give a rough third option. Stay off the roof if it is steep, wet or icy.
How much waste should I add for a hip roof?
About 15% is the usual allowance for hip roofs, versus 10% for a simple gable — every hip and valley angle generates cut shingles. Very cut-up roofs (dormers, multiple valleys) can need more; ordering one extra bundle beyond the calculation is cheap protection against a dye-lot mismatch later.