Rafter Calculator
Calculate rafter length from building span, roof pitch, and overhang. Find the number of rafters needed.
feet
inches
Rafter Length
—feet
Run (half span)—
Rise—
Pitch Multiplier—
Angle—
Rafter Length Formula
Rafter Length = √(Run² + Rise²) + Overhang. The run is half the building span. The rise equals run × (pitch/12).
Roof Pitch Multipliers
| Pitch | Multiplier | Angle |
|---|---|---|
| 3/12 | 1.031 | 14.0° |
| 4/12 | 1.054 | 18.4° |
| 5/12 | 1.083 | 22.6° |
| 6/12 | 1.118 | 26.6° |
| 8/12 | 1.202 | 33.7° |
| 10/12 | 1.302 | 39.8° |
| 12/12 | 1.414 | 45.0° |