Rafter Length Converter
Calculate rafter length from roof pitch and run.
Convert between pitch notation, degrees, and rafter length per foot of run.
Enter roof pitch and run to calculate rafter length.
Roof pitch describes how steep a roof is. It is expressed as the number of inches of rise per 12 inches of horizontal run (e.g., 6/12 means 6 inches of rise for every 12 inches of run).
The fundamental rafter length formula: Rafter length uses the Pythagorean theorem:
- Rafter Length = sqrt(Run^2 + Rise^2)
- Per foot of run: Rafter Length per foot = sqrt(12^2 + Pitch^2) / 12
For example, with a 6/12 pitch:
- Rafter per foot = sqrt(144 + 36) / 12 = sqrt(180) / 12 = 13.416 / 12 = 1.118 feet per foot of run
Complete rafter length table per foot of run:
| Pitch | Rise/Run | Degrees | Rafter per ft of Run | Rafter per ft (inches) |
|---|---|---|---|---|
| 1/12 | 1 in 12 | 4.8 | 1.003 ft | 12.04 in |
| 2/12 | 2 in 12 | 9.5 | 1.014 ft | 12.17 in |
| 3/12 | 3 in 12 | 14.0 | 1.031 ft | 12.37 in |
| 4/12 | 4 in 12 | 18.4 | 1.054 ft | 12.65 in |
| 5/12 | 5 in 12 | 22.6 | 1.083 ft | 13.00 in |
| 6/12 | 6 in 12 | 26.6 | 1.118 ft | 13.42 in |
| 7/12 | 7 in 12 | 30.3 | 1.158 ft | 13.89 in |
| 8/12 | 8 in 12 | 33.7 | 1.202 ft | 14.42 in |
| 9/12 | 9 in 12 | 36.9 | 1.250 ft | 15.00 in |
| 10/12 | 10 in 12 | 39.8 | 1.302 ft | 15.62 in |
| 11/12 | 11 in 12 | 42.5 | 1.357 ft | 16.28 in |
| 12/12 | 12 in 12 | 45.0 | 1.414 ft | 16.97 in |
Calculating total rafter length: Total rafter = Rafter per foot x Total run (in feet) + Overhang
- The “run” is the horizontal distance from the outside wall to the ridge (typically half the building width for a gable roof).
- Add the desired eave overhang (usually 12-24 inches / 30-60 cm).
- Add 6-12 inches for the ridge cut and birdsmouth cut waste.
Example calculation: A building is 24 feet wide with a 6/12 pitch and 18-inch eave overhang.
- Run = 24/2 = 12 feet
- Rafter per foot = 1.118
- Rafter length = 12 x 1.118 = 13.42 feet
- Add overhang: 1.5 x 1.118 = 1.68 feet
- Total = 13.42 + 1.68 = 15.10 feet (round up to 16 ft lumber)
Pitch to degrees conversion:
- Degrees = arctan(pitch / 12) x (180 / pi)
Common roof pitch ranges:
- Low slope: 1/12 to 3/12 (requires special roofing materials)
- Standard slope: 4/12 to 9/12 (most residential roofs)
- Steep slope: 10/12 to 12/12 (dramatic appearance, more expensive)