Roof Area Calculator
Turn your building footprint and pitch into true sloped roof area.
How this calculator works
roof area = length × width × √(rise² + 144) ÷ 12
Roof area is always larger than the footprint because the surface slopes. This calculation covers a simple gable. Hips, dormers and valleys add area that must be measured separately.
Worked example
With building length 40 ft, building width 30 ft, roof pitch (rise per 12) 6 in, this calculator returns 1341.64 sq ft.
Frequently asked questions
How do I calculate roof area from the ground?
Measure the building footprint, then multiply by the pitch multiplier. A 40 by 30 ft house with a 6/12 roof has 1,200 sq ft of footprint but 1,342 sq ft of actual roof.
What is a roofing square?
One hundred square feet of roof surface. Roofers quote and order in squares rather than square feet, and shingles are sold in bundles that cover a third of a square each.
Does roof area include overhangs?
It should. Measure the footprint to the outside edge of the eaves and rakes, not to the wall. A 12 inch overhang on all sides of a 40 by 30 house adds roughly 150 sq ft of roof.
How do I handle a hip roof or dormers?
Break the roof into simple planes and calculate each one separately, then add them. A hip roof has the same area as a gable of the same footprint and pitch, but dormers and valleys add area and waste.