Roofing Calculator: Squares, Bundles and Underlayment
Measure the building footprint and the pitch (or enter the roof area straight from the drawings), and get the roof area in squares, the number of shingle bundles after waste, the underlayment rolls, the ridge cap and a material cost estimate at your price per square.
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Read the how-to guide
The math
- Slope factor = √(1 + (rise ÷ 12)²). A 4:12 roof is 1.054, 6:12 is 1.118, 8:12 is 1.202, 12:12 is 1.414.
- Roof area = footprint length × width × slope factor. This works for a gable, a hip or any roof with one pitch, as long as the footprint includes the overhangs; it does not count dormers or the extra material in valleys, which the waste factor covers.
- One roofing square = 100 sq ft. Add the waste, multiply by bundles per square and round up to whole bundles.
- Underlayment rolls = squares with waste ÷ squares per roll, rounded up. Ridge cap = ridge and hip length ÷ about 33 ft per bundle when cut from 3-tab shingles.
- Cost = squares bought (bundles ÷ bundles per square) × price per square.
Starter strip, drip edge, ice-and-water shield, step and valley flashing, ridge vent, nails and dump fees are not included. For a tear-off, figure the disposal by the number of existing layers.
Worked example
A 40 × 30 ft footprint (1,200 sq ft) at 6:12 has a slope factor of 1.118, so the roof is 1,342 sq ft, or 13.42 squares. With 10% waste that is 14.76 squares; at 3 bundles per square, round up to 45 bundles (15 squares). Underlayment at 4 squares per roll takes 4 rolls, and 40 ft of ridge takes 2 bundles of cap. At $110 per square the shingles run about $1,650.
Frequently asked questions
How many bundles are in a square?
Three for standard 3-tab and most laminated architectural shingles (each bundle covers about 33 sq ft). Heavier designer and slate-look shingles can take four or five bundles per square, so check the wrapper.
How much waste should I add?
10% is typical for a simple gable roof. Hips, valleys, dormers, skylights and steep pitches generate more cut-offs, so use 15% or more. Starter courses can be cut from waste on 3-tab roofs but usually need a separate starter strip with architectural shingles.
How do I measure pitch from the ground?
Hold a level horizontally against the roof edge or a rafter tail, measure 12 in. along it, and measure the vertical distance down to the roof surface: that is the rise per 12. Use the roof pitch calculator to turn a measured angle into a pitch.
What does a square of shingles cost?
Prices vary by region and product. Basic 3-tab and standard architectural shingles are the lowest-cost materials per square; designer, impact-resistant and cool-roof shingles cost more. Get a current quote from your supplier and enter it.
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This calculator gives an estimate for planning and is not tax, legal or engineering advice. Confirm code-related results against the code edition adopted in your jurisdiction and with your authority having jurisdiction, and follow the manufacturer's instructions.