Roofing squares, shingle bundles, underlayment rolls, and nail counts from the building footprint, pitch multiplier, complexity factor, and waste allowance.
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FORMULA USEDRoofing Calculator
Squares: the roofer’s unit
Roofing materials sell by the square, which is 100 ft² of roof surface. Three bundles of standard 3-tab shingles cover one square (each bundle nets about 33.33 ft²), so bundles = squares × 3 before waste.
From footprint to real roof area
A pitched roof covers more surface than the flat footprint below it. The multiplier for a rise-per-12 pitch is √(144 + rise²) ÷ 12:
Pitch
4:12
6:12
8:12
10:12
12:12
Multiplier
1.054
1.118
1.202
1.302
1.414
Hips, dormers, and valleys add even more edge length, which is what the shape factor (1.00 to 1.20) approximates before the waste percentage pads the total.
Everything under the shingles
The board also counts underlayment rolls at 400 ft² of net coverage after laps, and fasteners at the common 320 nails per square (4 per shingle tab zone, adjusted per the product’s install sheet). Drip edge, ridge vent, and flashing are linear-foot items to measure separately.
Enter the building’s outside length and width — the flat footprint.
Set the pitch in rise per 12 inches, and pick the closest shape factor.
Keep waste at 10% for gables; raise it toward 15% for dormers and valleys.
Order bundles, underlayment, and nails from the rows, plus drip edge by the foot.
Frequently asked questions
What is a square in roofing?
One roofing square is 100 ft² of roof surface. Roofs are quoted in squares because shingles, underlayment, and labor are all priced per square rather than per foot of footprint.
How many bundles of shingles do I need per square?
Three bundles of standard 3-tab shingles cover one square. Heavier architectural lines also average three per square, but coverage varies by brand — check the bundle’s printed net coverage and adjust if it differs from 33.33 ft².
Does roof pitch change how many shingles I need?
Yes, and dramatically. The steeper the pitch, the longer the rafter run over the same footprint: a 12:12 roof needs about 41% more material than a flat one. The pitch multiplier handles this automatically.
How much waste should I add for a roof?
Plan on 10% for a simple gable and 15–18% for roofs with valleys, dormers, or skylights, where cuts multiply. Starter strips and ridge caps also come out of the waste allowance.
Should hips, valleys, and dormers be measured separately?
For an exact material order, yes — the shape factor here approximates them. Complex roofs reward climbing the ladder with a tape: measure each plane’s run and multiply by the rafter length per pitch.
How many nails and rolls of underlayment will the job use?
Figure 320 nails per square with standard 4-nail application, and one roll of underlayment per 400 ft² of roof after laps. Both appear as rows in the results for your adjusted area.