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Roofing Calculator
Enter the footprint and the pitch to get the true sloped area, in square feet and in squares.
The formula
multiplier = √(1 + (pitch ÷ 12)²)
roof area = footprint × multiplier × (1 + waste ÷ 100)
squares = roof area ÷ 100
Worked example
A 40 ft × 30 ft footprint at 6/12 pitch, 10% waste
- √(1 + (6 ÷ 12)²) = 1.1180
- 40 × 30 = 1,200 ft² of footprint
- 1,200 × 1.1180 = 1,341.6 ft² of roof
- 1,341.6 × 1.10 = 1,475.8 ft² → 14.76 squares
Where this goes wrong
Ordering shingles for the footprint instead of the slope
A roof is always bigger than the building under it. At 6/12 pitch it is 11.8% bigger; at 12/12 it is 41.4% bigger. Order for a 1,200 ft² footprint on a 12/12 roof and you are 500 ft² short — five squares, and a tarpaulin over the job while you wait.
Questions
- What is a roofing square?
- One hundred square feet of roof surface. It is a unit of trade rather than a measurement standard, and it is why roofing is quoted in numbers that look far too small for the size of a house.
- How do I find my pitch without climbing up?
- Hold a level out horizontally from the roof surface, mark 12 inches along it, and measure straight down from that mark to the roof. That drop is the rise per 12 of run — the first number in the pitch.
- How many bundles make a square?
- Three is the norm across the major manufacturers for standard asphalt shingles, at about 33⅓ square feet a bundle. Thicker architectural and designer shingles are often packed four to the square, so the figure is a field on this page rather than an assumption inside it.
- Why is the multiplier a square root?
- Because the slope is the hypotenuse of a right triangle whose base is 12 and whose height is the pitch. Pythagoras gives the sloped length, and dividing by the base turns it into a multiplier you can apply to any footprint.