The math behind your estimate
The inputs, allowance, and rounding stay visible.
- Footprint
- Plan area = (Length + 2 × overhang) × (Width + 2 × overhang)
- Multiplier
- Pitch multiplier = √(rise² + 12²) ÷ 12
- Surface
- Roof surface area = Plan area × pitch multiplier
- Angle
- Angle = arctan(rise ÷ 12), in degrees
- Squares
- Squares = Roof surface area ÷ 100 ft²
Assumptions
- A gable roof is assumed: two rectangular planes meeting at a ridge. Hips, valleys, dormers and turrets all add area — measure and add each additional section separately.
- Pitch is entered as rise per 12 inches of run, the convention roofers use. The equivalent angle in degrees and the multiplier are both shown so the conversion is never hidden.
- The overhang is applied to all sides of the footprint. If you are measuring the roof edges themselves rather than the walls, set the overhang to zero.
- A roofing square is 100 ft² of roof surface — a roofing-trade unit, not a measurement of your building.
Scope note
This tool estimates roof area for material planning only. It does not assess structural capacity, rafter sizing, snow or wind loading, ventilation or code compliance. Do not use it to decide whether a roof can carry a new covering — and measure from the ground or from inside the attic rather than climbing a steep roof.
Frequently asked questions
- Why is my roof area bigger than my house's square footage?
- Because a sloped roof is longer than the footprint it covers. A 4/12 pitch adds about 5.4%, a 6/12 adds 11.8%, and a 12/12 pitch adds 41.4%. Ordering materials off the footprint alone is the most common way people come up short.
- How do I find my roof pitch without getting on the roof?
- Measure inside the attic: hold a level out 12 inches horizontally from a rafter's underside, then measure straight down from the level's end to the rafter. That drop in inches is your rise per 12. A 4-inch drop is a 4/12 pitch.
- What is a roofing square?
- 100 square feet of roof surface. Roofers quote and order in squares, and shingles are sold in bundles that make up a square — commonly three bundles per square, though that varies by product.
- Does the calculator handle hip roofs?
- Not directly — it models gable roofs. A hip roof covering the same footprint at the same pitch has a very similar total surface area, so a gable calculation is a reasonable estimate; add extra for the additional cut waste hips and valleys create.
Sources
- Standard roof pitch multiplier table (rise/run slope factors)
- Roofing square convention (100 ft² per square)