Roof Area Calculator
Turn a building footprint and roof pitch into the true sloped roof area in square feet and roofing squares.
How it works
pitch multiplier = √(rise² + run²) ÷ run · roof area = footprint × multiplier · squares = area ÷ 100
A sloped roof covers more surface than the flat ground beneath it, so the footprint has to be scaled up by the pitch. The multiplier is the diagonal length of the rafter per unit of horizontal run — √(rise² + run²) ÷ run — where run is the standard 12. A 6/12 roof works out to about 1.118, so it holds nearly 12% more roofing than a flat one over the same footprint. Dividing the true area by 100 gives roofing squares, the unit shingles and labor are quoted in.
Worked example
A 40 × 30 ft footprint is 1,200 sq ft. At a 6/12 pitch the multiplier is √(6² + 12²) ÷ 12 = √180 ÷ 12 = 1.118, so the actual roof is 1,200 × 1.118 ≈ 1,342 sq ft — about 13.4 squares, or roughly 14 squares once you add 10% for waste.
Frequently asked questions
Why is the roof bigger than the footprint?
Because the roof rises as it spans. The steeper the pitch, the longer each rafter is for the same horizontal run, and the more surface there is to cover. A flat roof matches its footprint; a 12/12 roof needs about 41% more material for the same ground area.
What is a roofing square?
One hundred square feet of roof surface. Shingles, underlayment and labor are all priced by the square, so converting the true area to squares is the first step in any roofing estimate.
Does this account for hips, valleys and overhangs?
No — it uses a simple rectangular footprint times the slope multiplier. Overhangs, dormers, hips and valleys add area and cutting waste, so treat the result as a clean baseline and lean on the 10% waste margin, or measure complex roofs plane by plane.
Related calculators
Estimate for planning. Add a waste margin and confirm against your supplier specs and local building codes.