Roof Area Calculator

How to Calculate Roof Area

Your roof's surface area is larger than the area it covers, because the roof is tilted. To get the real surface area, multiply the footprint area by the pitch multiplier (also called slope factor).

Roof Area Formula

• Footprint Area = (Length + 2 × Overhang) × (Width + 2 × Overhang)
• Slope Factor = √(1 + (Rise / Run)²)
• Roof Surface Area = Footprint Area × Slope Factor

Worked Example

For a 50 ft × 30 ft roof with a 6/12 pitch: footprint = 1,500 sq ft, slope factor = 1.118, so the roof surface area ≈ 1,500 × 1.118 = 1,677 sq ft.

What You Need

The building's outside dimensions and the roof pitch. If you don't know the pitch, measure it with a level and tape measure in the attic, or use our Roof Pitch Calculator first.

Assumptions & Limitations

This calculator models a simple rectangular roof (gable/hip/shed/flat). Complex roofs with dormers, valleys or multiple sections need each section calculated separately and summed. Overhang, if entered, extends the footprint on all four sides.