Rafter Length Calculator (From Run and Roof Pitch)
Common rafter length, ridge rise and roof angle from the horizontal run and pitch, with optional overhang.
Formula
Pitch factor = √(1 + (rise/12)²). Rafter length = (run + overhang) × pitch factor. Ridge rise = run × rise ÷ 12. Angle = atan(rise ÷ 12).
Worked example
Inputs: Horizontal run (half the span, minus half the ridge board) = 14 ft; Pitch (rise per 12 in of run) = 6 in; Horizontal overhang (eave) = 12 in.
| Pitch factor | 1.118 |
|---|---|
| Rafter length ft | 16.77 |
| Rafter length no overhang ft | 15.65 |
| Ridge rise ft | 7 |
| Roof angle deg | 26.6 |
FAQ
What is the run of a rafter?
For a simple gable it is half the building span measured horizontally, minus half the ridge board thickness.
Is this the board length to order?
Add length for the plumb cut and tail cut, then round up to the next stock length.
What pitch is 6/12 in degrees?
atan(6 ÷ 12) ≈ 26.6 degrees.
Sources
- Geometry (volume/area formulas); no external constant needed
Related
- Roofing Calculator (Squares, Bundles & Pitch Factor)
- Stair Stringer Calculator (Risers, Treads & Stringer Length)
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Last reviewed 2026-10-13.