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.
Switch the site to metric — the control is in the footer — and your inputs, your results and the tables below follow. The formula, the constants, the guidance text and the sources stay in US units on purpose: that is how the standards they come from are written, and restating them would detach each figure from the document you are meant to be able to check it against.
Where these numbers come from
Geometry only — the sloped area is the footprint times the pitch multiplier. Nothing here is a published value.
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.
Roof area for houses of common footprint
Every row here opens the calculator already filled in. Copy that link and the numbers travel with it.
| Job | Size | True roof area |
|---|---|---|
| Small ranch, 4/12 | 40 ft × 26 ft × 4 in | 1,096 sq ft |
| Same house, 6/12 | 40 ft × 26 ft × 6 in | 1,163 sq ft |
| Same house, 9/12 | 40 ft × 26 ft × 9 in | 1,300 sq ft |
| Two-car garage | 24 ft × 22 ft × 5 in | 572.0 sq ft |
| Large two-storey | 50 ft × 32 ft × 8 in | 1,923 sq ft |
Simple gable only. Hips, dormers and valleys add area that has to be measured on its own.
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.