Roof Area Calculator
A roof area calculator multiplies footprint by √(rise² + 12²) ÷ 12: a 40 × 30 ft footprint at 6/12 pitch is 1,342 sq ft of roof, or 13.4 squares.
Roof area calculator: footprint times the pitch multiplier
To get roof area, take the flat footprint of the building and multiply it by a pitch multiplier. The multiplier is √(rise² + 12²) ÷ 12, where rise is the number of inches the roof climbs for every 12 inches of horizontal run. This roof area calculator does the multiplication and then divides by 100 to give roofing squares.
A 40 × 30 ft footprint at 6/12 pitch is 1,200 sq ft flat, but the roof surface is 1,342 sq ft, or 13.4 squares.
The formula in plain words
Picture a ladder leaning against a wall. The ground distance is the run. The height is the rise. The ladder itself is longer than either. The roof surface is the ladder, and the footprint is the ground.
For every 12 inches of run, the roof surface travels farther than 12 inches. At 6/12 that is √(36 + 144) = √180 = 13.42 inches, and 13.42 ÷ 12 = 1.118.
- Pitch multiplier = √(rise² + 144) ÷ 12
- Roof area = footprint area × multiplier
- Squares = roof area ÷ 100
Worked examples
- Gable roof, 40 ft × 30 ft footprint, 6/12 pitch
- Footprint: 40 × 30 = 1,200 sq ft
- Multiplier: √(6² + 12²) ÷ 12 = 1.118
- Roof area: 1,200 × 1.118 = 1,342 sq ft
- Squares: 13.4
- Steeper roof, 50 ft × 28 ft footprint, 8/12 pitch
- Footprint: 50 × 28 = 1,400 sq ft
- Multiplier: √(8² + 12²) ÷ 12 = 1.202
- Roof area: 1,400 × 1.202 = 1,683 sq ft
- Squares: 16.8
Shingles are commonly packaged three bundles to the square. That is about 41 bundles for example 1 before waste. Use the roofing shingle calculator for the bundle count with waste built in.
Pitch multiplier table
| Pitch | Multiplier | Roof area from 1,200 sq ft footprint |
|---|---|---|
| Flat (0/12) | 1.000 | 1,200 sq ft |
| 3/12 | 1.031 | 1,237 sq ft |
| 4/12 | 1.054 | 1,265 sq ft |
| 6/12 | 1.118 | 1,342 sq ft |
| 8/12 | 1.202 | 1,442 sq ft |
| 10/12 | 1.302 | 1,562 sq ft |
| 12/12 | 1.414 | 1,697 sq ft |
How to measure
- Footprint: Measure length and width at the eave line, including the overhang. If you only have wall dimensions, add the overhang on each side.
- Pitch: Place a level 12 inches along a rafter or the attic framing and measure the rise from the level to the rafter. The roof pitch calculator converts rise and run to a pitch number.
- Several sections: Run each section on its own, because a garage with a 4/12 roof next to a house at 8/12 needs two different multipliers.
- Dormers and porches: Add their areas as separate sections.
Common mistakes
- Ordering from the footprint. A 1,200 sq ft footprint at 6/12 is 142 sq ft short if you forget the multiplier.
- Measuring wall-to-wall only. Overhangs can add a foot or more on every side, and shingles cover them.
- Entering pitch as degrees. The field expects rise per 12 inches, so enter 6 for a 6/12 roof, not 26.6.
- Using one pitch for a cut-up roof. Different planes can have different slopes.
- Skipping waste. Hips, valleys, and cuts add leftovers that area alone does not capture. Allow 10 to 15 percent on busy roofs.
Why overhangs matter
The footprint you enter should include the eave overhang, because shingles cover it. Take the 40 × 30 ft house at 6/12. If the roof overhangs by 1 ft on every side, the roof footprint is 42 × 32 = 1,344 sq ft, not 1,200.
At the 1.118 multiplier, that is 1,503 sq ft of roof instead of 1,342. The overhang adds about 161 sq ft, or 1.6 squares. Leave it out and you can come up a bundle or more short.
Related calculators
Once you have the area, price the job with the roofing cost calculator and size the underlayment with the roofing underlayment calculator. For a hipped design, the hip roof calculator breaks out ridge and hip lengths.
Frequently asked questions
Why is roof area larger than my house footprint?
A sloped roof covers more surface than the flat outline beneath it. The steeper the pitch, the bigger the gap: a 6/12 roof is about 11.8 percent larger than its footprint, while a 12/12 roof is 41.4 percent larger. This calculator applies that pitch multiplier automatically.
What is a roofing square?
One square equals 100 square feet of roof surface. Shingles, underlayment, and labor are all quoted in squares, so the calculator converts your area into squares as well as square feet. A 1,342 sq ft roof is 13.4 squares.
Do I measure the footprint at the ground or the eaves?
Measure the roof's horizontal footprint including any eave overhangs, since the roof surface extends over them. Measure at the eave line, not the wall line, for an accurate area.
How do I handle a complex roof with multiple sections?
Break the roof into simple rectangular sections, calculate each section's footprint and pitch separately, and add the areas. Hips, valleys, and dormers also add waste, so plan roughly 10 to 15 percent extra material for cut-up roofs.
Does a hip roof have a different area than a gable roof?
Not if the pitch is the same on every plane. A hip roof and a gable roof over the same footprint at the same pitch have the same surface area, because the multiplier applies to the whole footprint. Hips mainly add cutting waste and ridge length.
Estimates only. Verify quantities with your supplier before purchasing.