How to Calculate Roof Square Footage and Roofing Squares
Multiply the roof’s footprint (including overhangs) by the pitch factor: a 40 ft × 30 ft footprint with a 6/12 pitch gives 1,200 × 1.118034 = 1,341.64 square feet, or 13.4 roofing squares.
The pitch factor is √(1 + (rise ÷ 12)²). Roofers count in squares of 100 sq ft, so with 10% waste on a simple gable roof, plan on about 14.8 squares.
How do you calculate roof square footage in four steps?
- Measure the footprint. Take the length and width of the roof as seen from above, including the eave and rake overhangs, and multiply. Do this from the ground or from drawings. For a roof built from several parts, measure each part and add.
- Find the pitch. Pitch is written as rise over 12, such as 6/12.
- Look up or compute the pitch factor. It is the slope length divided by the horizontal run:
factor = √(1 + (rise ÷ 12)²). - Multiply and convert. Roof area = footprint × factor. Squares = roof area ÷ 100.
The factor is needed because a sloped surface is larger than the flat area under it. The steeper the roof, the bigger the gap.
How do you find the roof pitch?
Look for it on the plans. Otherwise, from the attic, hold a level horizontal against a rafter, mark 12 inches along the level and measure straight up from that mark to the rafter. That vertical distance in inches is the rise in rise/12. You can also work back from an angle or percent grade with the roof pitch calculator. Measuring from inside or from the ground avoids climbing a sloped roof. The guide on how to calculate roof pitch shows three safe ways to measure it.
Pitch factor table
| Pitch | Pitch factor | Angle | Roof area for a 1,200 sq ft footprint |
|---|---|---|---|
| 2/12 | 1.0138 | 9.5° | 1,217 sq ft |
| 3/12 | 1.0308 | 14.0° | 1,237 sq ft |
| 4/12 | 1.0541 | 18.4° | 1,265 sq ft |
| 5/12 | 1.0833 | 22.6° | 1,300 sq ft |
| 6/12 | 1.1180 | 26.6° | 1,342 sq ft |
| 7/12 | 1.1577 | 30.3° | 1,389 sq ft |
| 8/12 | 1.2019 | 33.7° | 1,442 sq ft |
| 9/12 | 1.2500 | 36.9° | 1,500 sq ft |
| 10/12 | 1.3017 | 39.8° | 1,562 sq ft |
| 12/12 | 1.4142 | 45.0° | 1,697 sq ft |
Worked example: a 40 ft × 30 ft gable roof, 6/12
The footprint, including overhangs, is 40 × 30 = 1,200 sq ft.
- Pitch factor: √(1 + (6 ÷ 12)²) = √1.25 = 1.118034
- Roof area: 1,200 × 1.118034 = 1,341.64 sq ft
- Squares: 1,341.64 ÷ 100 = 13.416 squares
- With 10% waste: 13.416 × 1.10 = 14.758 squares, so order 15 squares of coverage
Cross-check by planes. A gable roof has two planes. Each spans 15 ft of run (half of the 30 ft width). The slope length is 15 × 1.118034 = 16.7705 ft. One plane is 40 × 16.7705 = 670.82 sq ft, and two planes give 1,341.64 sq ft. Same answer.
Enter the footprint and pitch, choose a waste percentage, and see roof area, squares and bundles.
How much waste should you add to a roof?
Waste covers cut-offs at edges, valleys and hips, plus the odd damaged shingle. Common starting points are 10% for a simple gable roof and 15% for a hip roof or one with valleys and dormers. Sources disagree (gables are often quoted at 5% to 10% and hips at 10% to 25%), so treat these as conventions and follow the manufacturer’s instructions. For the example roof, 15% gives 13.416 × 1.15 = 15.43 squares, so 16.
- Hip roofs. If every plane has the same pitch, a hip roof over the same footprint has the same area as a gable roof. It just creates more cuts, so use a higher waste percentage.
- Dormers and additions. Treat each as extra planes and add their areas.
- Mixed pitches. Compute each section with its own factor and add the results.
From squares you can estimate bundles. Many shingles come 3 bundles to the square, but some products use 4 or 5, which changes the count a lot: 14.758 squares is 45 bundles at 3 per square and 60 at 4. The details are in how many shingles are in a bundle. Starter strip, ridge cap, underlayment and drip edge are bought by length or roll, not by roof area.
Checking a roofing quote
For this example roof, you would expect about 13.4 squares of actual roof and about 15 squares of material. If a quote lists far more, ask whether the number includes waste, how many layers are being torn off, and whether it covers extras such as dormers or an attached garage. For the flat-area basics behind the footprint, see how to calculate square feet.
Assumptions and safety
- The method assumes flat planes and that the footprint includes overhangs. Measuring only the house walls understates the area.
- It is a quantity estimate, not roof design. Decking, ventilation, wind and fire ratings, and minimum slope for a given shingle are set by the manufacturer and local code.
- A near-flat roof is a different product category. Shingles have minimum slopes in the manufacturer’s instructions, so check before you buy.
- Roofs are fall hazards. Prefer measuring from the ground, plans or the attic, and follow OSHA guidance if you work on one.
Questions people ask
How many square feet is one roofing square?
100 sq ft of roof surface. The example roof, 1,341.64 sq ft, is 13.42 squares.
Is roof square footage the same as the house’s square footage?
No. The roof is larger because it slopes and overhangs the walls. A 1,200 sq ft footprint is 1,217 sq ft of roof at 2/12, 1,342 sq ft at 6/12 and 1,697 sq ft at 12/12.
How many squares of shingles do I need for a 1,500 sq ft roof?
1,500 ÷ 100 = 15 squares. With 10% waste that is 16.5 squares, or 50 bundles at 3 bundles per square (16.5 × 3 = 49.5).
Does a hip roof have more square footage than a gable roof?
Not if every plane has the same pitch and the footprint is the same: the area is identical. A hip roof makes more cuts, so use a higher waste percentage.
How do I measure a roof without climbing on it?
Measure the footprint from the ground or the plans, including the overhangs, and the pitch from the attic or the plans. Roofs are fall hazards; follow OSHA guidance if you work on one.
Sources
- Owens Corning, The Roofing Collection Product Guide. Coverage per square and bundles per square by product. Accessed October 16, 2026.
The pitch factor is plane geometry (the Pythagorean theorem). Waste percentages are industry conventions, not a standard.