Roofing Calculator

A roofing square is 100 ft² of roof surface. Divide your total sloped roof area by 100 to get squares; a 2,000 ft² roof is 20 squares.

Pitch increases the sloped area above the flat footprint. A 6/12 pitch has a slope factor of about 1.118, so it needs roughly 12% more material than the footprint alone.

Most architectural asphalt shingles come three bundles to the square (100 ft²), so a 20-square roof needs about 60 bundles plus waste.

Pitch is the rise over 12 inches of horizontal run. Hold a level against the roof, mark 12 inches along it, and measure straight down from that mark to the roof surface. If it drops 6 inches, the roof is a 6/12 pitch. You can measure this from inside the attic against a rafter.

Take the arctangent of the rise divided by 12. A 4/12 pitch is 18.4°, 6/12 is 26.6°, 9/12 is 36.9° and 12/12 is exactly 45°. Roofers quote pitch as a ratio because it is what you can measure on the roof; degrees are mostly for solar and framing calculations.

Measure the building footprint, then multiply by the slope factor for your pitch — the roof surface is always larger than the ground it covers. The factor is √(rise² + 144) ÷ 12: about 1.118 at 6/12. A 40 by 25 footprint at 6/12 is 1,000 × 1.118, or roughly 1,118 ft².

Divide the roof area in square feet by 100. One roofing square is 100 ft² of finished roof surface — the unit suppliers quote and estimators bid in. A 1,800 ft² roof is 18 squares. Use the sloped area, not the footprint, or you will under-order by 10–30%.

Work in squares, then bundles: three bundles cover one square for standard three-tab and most architectural shingles. Add 10% for waste, plus extra for hip and ridge caps and for starter course. A 20-square roof needs about 60 bundles before waste, 66 with it.