CALCULATION METHOD
How to calculate roof pitch
Roof slope is the vertical rise divided by the horizontal run. In US construction it is commonly restated as inches of rise for every 12 inches of run. A roof that rises 6 inches over 12 inches has a 6:12 pitch.
Pitch rise = rise ÷ run × 12
Angle = arctan(rise ÷ run)
Percent slope = rise ÷ run × 100
Multiplier = √(1 + (rise ÷ run)²)
Rafter length = horizontal run × multiplier
Roof pitch calculator example
A 6-inch rise over a 12-inch run is a 6:12 pitch. Its angle is about 26.57°, its percent slope is 50%, and its multiplier is 1.1180. For a 24-foot-wide simple gable with a 12-inch horizontal overhang, the sloped distance from rafter tail to ridge is about 14 ft 6.4 in before construction deductions.
Roof pitch angle chart
Use this chart for quick reference. The calculator above accepts nonstandard and fractional pitches too.
| Pitch | Angle | Percent slope | Multiplier |
|---|---|---|---|
| 2:12 | 9.46° | 16.7% | 1.0138 |
| 3:12 | 14.04° | 25.0% | 1.0308 |
| 4:12 | 18.43° | 33.3% | 1.0541 |
| 5:12 | 22.62° | 41.7% | 1.0833 |
| 6:12 | 26.57° | 50.0% | 1.1180 |
| 8:12 | 33.69° | 66.7% | 1.2019 |
| 10:12 | 39.81° | 83.3% | 1.3017 |
| 12:12 | 45.00° | 100.0% | 1.4142 |
Rise, run, span, and rafter length
Rise is vertical; run is horizontal. On a simple symmetrical gable roof, the run from the outside wall to the ridge centerline is half the building span. The sloped side of that right triangle is the common-rafter line. An actual cut length still needs project-specific deductions and layout details.
GAF Steep-Slope Pro Field Guide for rise/run terminology and Johnson Level construction calculator manual for pitch conversion and common-rafter examples. Accessed September 11, 2026.