Roof surface area and roofing squares needed, from the building footprint and roof pitch.
Pitch is entered as the rise per 12 inches of horizontal run — a 6:12 pitch rises 6 inches for every 12 inches of run, giving a pitch multiplier of √(1 + (6/12)²) ≈ 1.118. A steeper pitch means more actual roof surface than the flat building footprint.
A square is roofing-industry shorthand for 100 square feet of roof area. The calculator divides your total roof area (with waste included) by 100 to get the square count, which is how shingles and underlayment are typically priced and ordered.
Standard asphalt shingles are packaged at 3 bundles per square, so the calculator multiplies the roofing squares (with waste) by 3 and rounds up, giving you a bundle count ready to take to the supplier.