The formula
Pitch (x/12) = rise ÷ run × 12
Angle = arctan(rise ÷ run)
Rise and run must be in the same unit. The result is the x in x/12, so a roof that rises 6 inches over 12 inches of run is 6/12, which is 26.57 degrees.
Method 1: from a rise and run measurement
Say a level held 24 inches out from the rafter shows 7 inches of rise. Pitch = 7 ÷ 24 × 12 = 3.5, so the roof is 3.5/12 and 16.3 degrees. Round to the nearest whole number only if you are matching a standard pitch: here that is 3.5/12, which is not a standard pitch, so check the measurement before you order anything.
Method 2: from an angle in degrees
A phone angle app reads 22 degrees. Pitch = tan(22°) × 12 = 0.404 × 12 = 4.85, so the roof is about 4.8/12, which is close to 5/12 (22.6°). The degrees to pitch chart does this lookup for every angle.
Method 3: from the rafter length and the run
You cannot reach the ridge, but you can measure a rafter end to end and the horizontal run beneath it. Take a rafter 18.0 ft long over a run of 16.0 ft. The rise is √(18² − 16²) = √68 = 8.25 ft. Pitch = 8.25 ÷ 16 × 12 = 6.18, about 6.2/12.
Method 4: from a percent slope
Drawings for low-slope roofs often give percent. A 40% slope means 40 units of rise per 100 of run, so pitch = 40 ÷ 100 × 12 = 4.8/12. To go back, divide the pitch by 12: a 6/12 roof is a 50% slope.
Which method to use
| You have | Use | Example result |
|---|---|---|
| A level and a tape in the attic | Rise and run | 7 in over 24 in = 3.5/12 |
| A phone or angle finder | Degrees | 22° = 4.8/12 |
| A rafter and its run | Rafter length | 18 ft over 16 ft = 6.2/12 |
| A drawing in percent | Percent slope | 40% = 4.8/12 |
Skip the arithmetic with the roof pitch calculator, which takes rise and run or an angle. For ways to get the measurement itself safely, see how to measure roof pitch, and for the terms, what roof pitch means.
Frequently asked questions
How do you calculate roof pitch?
Divide the rise by the run, then multiply by 12. A rise of 6 inches over 12 inches of run is a 6/12 pitch.
How do you convert degrees to roof pitch?
Take the tangent of the angle and multiply by 12. For 30 degrees that is 6.93/12.
How do you find pitch from rafter length?
Subtract the squared run from the squared rafter length, take the square root to get the rise, then divide rise by run and multiply by 12.