Roofing Calculator
Use this roofing calculator to estimate roof area, roofing squares and shingle bundles from your building size and roof pitch. Enter the footprint and overhang (or a plan area you already know), the pitch as rise per 12, a waste percentage and, if you like, a price to see the cost. Bundles per square and underlayment coverage are typical values you can change.
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Plan area with overhang
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Slope factor
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Roof surface area
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Area to buy (with waste)
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Roofing squares (before waste)
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Roofing squares (with waste)
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Shingle bundles (rounded up)
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Underlayment rolls (rounded up)
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Ridge and hip cap length (approx.)
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Estimated cost
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Plan area = (length + 2 × overhang) × (width + 2 × overhang). Roof surface = plan area × √(1 + (pitch ÷ 12)²), which assumes the same pitch on every plane, so gable, hip and shed roofs give the same area; the shape only changes the cap length and the usual waste. 1 roofing square = 100 sq ft. Bundles = squares with waste × bundles per square, rounded up. Waste of 10% (15% for hip roofs), 3 bundles per square and 4 squares per underlayment roll are typical starting values, so check your product's label. Dormers, valleys, chimneys, skylights, starter strips and drip edge are not counted, and the cap length is an approximation for a plain rectangular roof.
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How it works
- How do I calculate roof area from the building size?
- Add the overhang to each side of the footprint to get the plan area, then multiply by the slope factor √(1 + (pitch ÷ 12)²). A 40 × 30 ft building with a 1 ft overhang is 42 × 32 = 1,344 sq ft in plan. At 6/12 the factor is 1.118, so the roof surface is about 1,502.6 sq ft.
- How many bundles of shingles are in a roofing square?
- One roofing square covers 100 sq ft. Many 3-tab and architectural shingles are sold at 3 bundles per square, but some products need 4 or more, so check the label. Continuing the example: 1,502.6 sq ft plus 10% waste is 1,652.9 sq ft, or 16.53 squares. That is 16.53 × 3 = 49.6, so 50 bundles.
- How much waste should I add for roofing?
- 10% is a common starting point for a simple gable roof, and about 15% is typical guidance for hip roofs, which have more cuts at the hips and ridges. Roofs with valleys and dormers can need more. The calculator lets you enter any percentage.
- How do I find my roof pitch?
- Pitch is the rise in inches for every 12 inches of horizontal run, so 6/12 rises 6 in per 12 in. Measure it with a level and a tape, or use the roof pitch calculator to convert rise, run or an angle. Slope factors: 4/12 = 1.054, 6/12 = 1.118, 8/12 = 1.202, 12/12 = 1.414.
- How many rolls of underlayment do I need?
- Divide the squares by the squares one roll covers. This tool defaults to 4 squares per roll, a typical figure, but coverage varies with the product and its required overlap, so enter the number on your roll's label. For 16.53 squares at 4 per roll that is 4.13, rounded up to 5 rolls.