How it works
The calculator adds the overhang to both sides of the footprint, then multiplies that area by the slope factor for your pitch. The slope factor turns level distance into distance along the roof surface.
Footprint incl. overhang = (Length + 2 × Overhang) × (Width + 2 × Overhang)
Slope factor = √(1 + (Rise ÷ Run)²)
Roof area = Footprint incl. overhang × Slope factor
Squares = Roof area ÷ 100 sq ft
This is exact for any roof whose planes all share one pitch: a gable or a hip roof of the same footprint and pitch has the same surface area. A pitch entered as an angle is converted to rise ÷ run first.
Worked example
Inputs
- Building length
- 40 ft
- Building width
- 30 ft
- Overhang
- 1 ft
- Roof pitch
- 6:12
- Roof type
- Gable (two sloped sides)
Result
- Footprint incl. overhang
- 1,344 sq ft
- Slope factor
- 1.118
- Roof angle (°)
- 26.6
- Roof area
- 1,503 sq ft
- Roofing squares
- 15.03 squares
Common mistakes
- Measuring the house without the overhang. Eaves and rakes add roof area on every side; enter the overhang so it is included.
- Using the footprint area as the roof area. A sloped roof is always larger than the ground it covers – the steeper the pitch, the bigger the difference.
- Mixing up pitch and angle. 6:12 is a ratio (rise over run), not 6 degrees. Type the ° sign if you are entering an angle.
- Treating a roof with several pitches as one. If the main roof and a porch roof have different pitches, calculate each section separately.
Assumptions and limits
- The footprint is a rectangle and the overhang is the same on all four sides.
- All roof planes share one pitch. For such roofs the result is exact for gable and hip shapes alike, because every plane’s true area is its footprint × the same slope factor.
- Dormers, valleys between different pitches, chimneys, skylights and multi-level roofs are not accounted for.
- The result is surface area only. It does not include waste, ridge or hip caps, starter strips, underlayment overlaps or any other material allowance.
Frequently asked questions
What is a roofing square?
A square is 100 sq ft of roof surface. Shingles and other roofing are often sold and priced by the square, so the calculator shows both the area and the number of squares.
Why is the result the same for a gable and a hip roof?
When all planes have the same pitch, the sloped area depends only on the footprint and the pitch. A hip roof has more edges and a different shape, but the same total surface. The roof type changes the drawing, not the area – though a hip roof does need more hip cap material.
What is the slope factor?
It is the number you multiply the footprint by to get the sloped area, √(1 + (rise ÷ run)²), which follows from Pythagoras. It gives 1.054 for 4:12, 1.118 for 6:12 and 1.414 for 12:12; IKO and Owens Corning both list 1.118 for a 6:12 roof in their estimating guides.
How do I find my roof pitch?
Check the building plans, or measure from inside the attic or at a gable end – how many inches the rafter rises over 12 in of level distance. A phone level app that reads angles also works; enter the result in degrees.
Sources
- Estimating How Much Roofing Is Required — IKO. Actual area = horizontal area × area/rake factor; 6/12 → 1.118. The factor equals √(1 + (rise/run)²) (Pythagoras).
- DIY Roofing: How to Measure a Roof — Owens Corning. "Plane Area x Correction Factor = Roof Area"; slope correction factor 6:12 → 1.118.
Method reviewed November 2, 2026