Roof Pitch Calculator
Enter a pitch as rise-per-12 or in degrees and get every other form of it — angle, percent grade, slope factor — plus common rafter length and the true roof surface area for ordering material.
Three ways to say the same slope
A roof slope can be written as a ratio, an angle or a percentage, and different trades default to different ones. Roofers and framers use rise-per-12. Engineers and solar installers use degrees. Drainage and gutter specs use percent grade.
- Angle = arctan(rise ÷ 12)
- Percent = rise ÷ 12 × 100
- Slope factor = √(rise² + 12²) ÷ 12
Common rafter length is half the span times the slope factor, plus the overhang times the same factor. True roof area is the flat footprint times the slope factor — the single most useful number on this page, because it is what you order shingles against.
Pitch reference table
| Pitch | Angle | Grade | Slope factor | Notes |
|---|---|---|---|---|
| 2/12 | 9.46° | 16.7% | 1.014 | Shingle minimum |
| 4/12 | 18.43° | 33.3% | 1.054 | Standard underlayment |
| 6/12 | 26.57° | 50.0% | 1.118 | Walkable limit |
| 8/12 | 33.69° | 66.7% | 1.202 | Roof jacks needed |
| 12/12 | 45.00° | 100% | 1.414 | Fall arrest |
Measuring pitch on an existing roof
From inside the attic, hold a level horizontally against a rafter, mark 12 inches along it, and measure straight up from that mark to the underside of the rafter. That vertical measurement is the rise. Doing it in the attic avoids both the ladder and the shingle thickness, which distorts the measurement on the outside.
Frequently asked questions
What does a 6/12 roof pitch mean?
It means the roof rises 6 inches vertically for every 12 inches of horizontal run. Pitch is always expressed against a 12 inch run in North America, so the first number is the only one that changes. A 6/12 pitch works out to 26.57 degrees, or 50 percent slope.
How do I convert roof pitch to degrees?
Take the arctangent of rise divided by run. For a 6/12 pitch that is arctan(6/12) = 26.57 degrees. The relationship is not linear, which is why a 12/12 roof is 45 degrees rather than double the angle of a 6/12 roof at 26.57 degrees.
What is the slope factor and why does it matter?
The slope factor is how much larger the actual roof surface is than the flat footprint beneath it. A 6/12 roof has a factor of 1.118, so a 1,000 square foot footprint carries 1,118 square feet of roofing. Ordering shingles from the building's floor area instead of the roof area is the classic way to come up short.
What is the minimum pitch for asphalt shingles?
Asphalt shingles require a minimum 2/12 pitch, and between 2/12 and 4/12 manufacturers require a double layer of underlayment for the warranty to hold. Below 2/12 you need a membrane roof — rolled, TPO or modified bitumen — because water moves too slowly to clear the laps.
At what pitch does a roof stop being walkable?
Most roofers treat 6/12 as the practical limit for working without staging. From 7/12 upward, roof jacks and planks become necessary, and above 9/12 the job typically needs full fall arrest. Pitch drives labour cost far more than it drives material cost.
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