Roofing Calculator
Estimate the actual sloped area of a roof from the building's footprint and its pitch — the flat footprint always understates it. Includes a waste allowance and the count in roofing "squares".
How to use this calculator
- Enter the building's footprint length and width in metres.
- Choose the roof pitch — the slope, given as rise/run (6/12) or angle.
- Use 12% waste for simple roofs, 15–20% for roofs with valleys and dormers.
Formula used
The pitch factor accounts for slope: a 6/12 roof (6 units rise per 12 run) has factor √(6² + 12²)/12 = 1.118, so it's 11.8% larger than the flat footprint. Steeper roofs have bigger factors — a 45° roof is 41% larger than its footprint.
Example calculation
A 12 m × 8 m building (96 m² footprint) with a 6/12 roof: 96 × 1.118 = 107.3 m² of actual roof.
With 12% waste, order for 120 m² — about 12.9 US roofing squares. Estimating from the 96 m² footprint would leave you 24 m² short.
Why footprint underestimates the roof
A pitched roof covers more area than the building beneath it, because the sloped surface is the hypotenuse of a triangle whose base is the footprint. The steeper the pitch, the larger the multiplier — gentle roofs add a few percent, a 45° roof adds over 40%. Ordering shingles or tiles off the flat footprint is a classic way to come up dramatically short.
This calculator assumes a simple gable or hip roof. Real roofs with valleys, dormers, hips and penetrations generate far more cut waste and complex geometry, which is why professional roofers measure directly and carry 15–20% waste. And roofs should never be measured by climbing on them — ground measurement or drawings are both safer and, with the pitch factor, accurate.
Why use this calculator?
- Convert footprint to true sloped area — the number materials are sold against.
- Pitch factors built in for common roof slopes.
- Get the count in roofing squares for US shingle ordering.
Frequently asked questions
How do I calculate roof area from the ground?
Measure the building's footprint (length × width), then multiply by the pitch factor for your roof's slope. A 6/12 pitch multiplies by 1.118. This avoids ever getting on the roof, which is both safer and accurate.
What is a roofing square?
A US roofing unit equal to 100 square feet (about 9.29 m²). Shingles are sold and estimated in squares; a typical bundle covers about a third of a square. The calculator converts your area automatically.
How much waste for roofing?
12% for simple gable roofs, 15–20% for complex roofs with valleys, hips and dormers where cuts multiply. Starter courses, ridge caps and overlaps also consume extra material beyond the plain area.