Roofing is sold by the square — 100 ft² of roof — and shingles come in bundles, usually three bundles to a square. This calculator turns your building's footprint and roof pitch into true roof area, squares, bundles and underlayment rolls, with a cutting-waste allowance.
The formula
The term under the root is the slope factor: it stretches the flat footprint into the sloped surface. A 4/12 pitch stretches by 1.054, a 6/12 by 1.118, an 8/12 by 1.202, and a 12/12 (45°) roof by 1.414. Don't forget the overhangs — a 12 in eave all round adds real area: our worked example's roof grows by more than 200 ft² because of them.
Finding your pitch
Put a level 12 inches out from the roof edge (or use a phone inclinometer app on the roof surface) and measure the vertical rise over that 12 inch run. Gable ends and attic access points are the safest places to measure. Pitch is written rise/12, so 6/12 means the roof climbs 6 inches for every 12 inches across.
What else a roof order needs
Bundles in this calculator cover the field of the roof. A complete order also wants starter-strip shingles along eaves and rakes, ridge cap shingles (about 35 linear ft per bundle of caps), underlayment (calculated here), drip edge, flashing, and roughly 2¼ lb of roofing nails per square. Ice-and-water shield is code-required along eaves in cold climates — typically one 6 ft band, about 200 ft² per roll.
Worked example
A 30 ft × 40 ft house with 12 in overhangs and a 6/12 pitch: footprint with overhangs = 32 × 42 = 1,344 ft²; slope factor for 6/12 = 1.118; roof area = 1,503 ft² = 15.03 squares. At 3 bundles per square with no waste that is 46 bundles; with a 10% allowance you order 50 bundles and 5 rolls of underlayment (400 ft² rolls).
How to use this tool
- Measure the building's length and width at ground level (or from plans).
- Add the eaves overhang on each side — measure it, don't guess.
- Select the roof pitch; measure rise over a 12 inch run if you are unsure.
- Choose bundles per square from the shingle packaging and set your waste allowance.
- Order bundles and underlayment from the results, then add starter, ridge cap and flashing separately.
Waste depends on the roof, not just the cuts
Simple gable roofs waste about 10%. Hip roofs push toward 12–15% because every course ends in an angled cut, and roofs with many valleys and dormers can exceed 15%. Returning an unopened bundle is easy at most suppliers — being one bundle short on the last Saturday of the job is not.