How to use the roof pitch calculator
- Measure the run: the horizontal distance from the outside of the wall to the point below the ridge. On a symmetrical gable roof that is half the building width.
- Measure the rise: how far the ridge sits above the top of the wall.
- Enter both values. The angle, the slope in percent, the x/12 pitch and the rafter length appear at once. You can also type an angle or a percentage and the other fields follow.
Any length unit works as long as rise and run use the same one. The unit menu only changes the label.
Degrees, percent and x/12
In the US and Canada roof pitch is usually written as x/12: inches of rise per 12 inches of run. A 6/12 roof rises 6 inches for every foot, which is 26.57° or a 50 % slope. Elsewhere the angle in degrees is more common.
| Convert | Formula |
|---|---|
| Rise and run → angle | angle = arctan(rise ÷ run) |
| Angle → percent | percent = tan(angle) × 100 |
| Angle → x/12 | x = tan(angle) × 12 |
| Rafter length | run ÷ cos(angle) = √(run² + rise²) |
A 100 % slope is not a vertical wall but a 45° roof: 12/12, rise equals run.
Roof pitch chart
Common pitches with their angle and slope, rounded.
| Pitch | Angle | Slope |
|---|---|---|
| 1/12 | 4.8° | 8.3 % |
| 2/12 | 9.5° | 16.7 % |
| 3/12 | 14° | 25 % |
| 4/12 | 18.4° | 33.3 % |
| 5/12 | 22.6° | 41.7 % |
| 6/12 | 26.6° | 50 % |
| 7/12 | 30.3° | 58.3 % |
| 8/12 | 33.7° | 66.7 % |
| 9/12 | 36.9° | 75 % |
| 10/12 | 39.8° | 83.3 % |
| 11/12 | 42.5° | 91.7 % |
| 12/12 | 45° | 100 % |
The rafter length from the calculator runs from the wall line to the ridge. Add the overhang and subtract half the ridge board thickness for the cut length.
Frequently asked questions
How do I find the pitch without getting on the roof?
Measure the rise and run at a gable end or in the attic, or hold a level against the underside of a rafter and measure how far it drops over 12 inches.
What is a low-slope roof?
Roofs below about 3/12 (14°) are usually called low-slope. Many shingle products need a minimum pitch and extra underlayment on shallow roofs; the manufacturer's instructions set the limit.
Is 100 % the steepest possible slope?
No. 100 % equals 45°. Steeper roofs have slopes above 100 %, and a vertical surface would be infinite.