Rafter Length Calculator
Select pitch, enter building span and overhang — get exact rafter line length and cut angles instantly.
Common Pitch Reference Table
| Pitch | Angle | Slope % | Multiplier | Plumb Cut | Level Cut | Type |
|---|
How Rafter Length Is Calculated
The rafter line length (L) is the distance along the rafter from the ridge to the wall's top plate — not the full board length. It's found using the Pythagorean theorem: L = √(Run² + Rise²), or equivalently, run × the pitch multiplier.
Pitch Multiplier
Each pitch has a multiplier (√(1 + (rise/run)²)) that converts horizontal run into rafter length. For a 6:12 pitch the multiplier is 1.118 — meaning for every 12" of run, the rafter is 13.42" long. Multiply the building's half-span by the multiplier to get the line length.
Overhang
The overhang (S) extends the rafter past the plate. The along-rafter overhang length = horizontal overhang × pitch multiplier. This is added to the line length to get the full board cut length before the plumb (tail) cut is made.
Plumb and Level Cuts
The plumb cut (at the ridge) is vertical — its angle from vertical equals the pitch angle. The level (seat) cut at the birdsmouth is horizontal. Both angles are derived from the same pitch ratio: plumb cut = atan(rise/run), level cut = 90° minus plumb cut.