About this tool
Roofing Calculator is a free, in-browser tool that turns a building's footprint and roof pitch into the actual sloped roof area, the number of roofing squares, and how many shingle bundles to order. All figures are computed locally as you type — nothing is uploaded.
A sloped roof covers more area than the footprint beneath it. The slope factor is √(pitch² + 144) ÷ 12, where pitch is the rise in inches per 12 inches of run, so a 6-in-12 roof has a factor of about 1.118. Roof area is footprint (length × width) × slope factor × (1 + waste ÷ 100). One roofing square is 100 square feet, and standard three-tab shingles come three bundles to the square, so bundles = roofing squares × 3, rounded up.
Use it to budget a re-roof: enter the footprint of the area under the roof, set the pitch, and the waste percentage (10% by default) covers hips, valleys, starter and ridge caps so the bundle count is a safe order quantity.
Frequently asked questions
What is the slope factor and why does it matter?
It converts flat footprint area to true sloped area. For a pitch of rise-in-12, the factor is √(pitch² + 144) ÷ 12. A steeper roof has a larger factor and therefore more surface to cover than its footprint suggests.
What is a roofing square?
A square is 100 square feet of roof surface — the standard unit shingles are sold and priced by. The tool divides the total roof area by 100 to give squares to one decimal place.
How many bundles are in a square?
Most architectural and three-tab asphalt shingles are packaged three bundles per square, so the tool multiplies squares by three and rounds up. Check your specific product, since some heavy shingles run four or five bundles per square.
Does the footprint include overhangs?
Enter the footprint of the area actually under the roof, including eaves and overhangs, then let the slope factor and waste percentage do the rest. Everything is calculated in your browser with nothing uploaded.
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