Rafter Length Calculator
From a building span and roof pitch, lay out the common rafter — run, rise, line length, the plumb and seat (birdsmouth) cut angles, and the sloped overhang.
Total width across the wall plates.
Rise per 12" of run. Accepts 6, 7.5, or a ratio like 6:12.
Horizontal projection past the wall.
The rafter is shortened by half of this.
- Common rafter runs at 26.6° — mark the plumb cut with a framing square set to 6 (rise) on the tongue and 12 (run) on the body.
- Line length is 13.35 ft from the ridge to the birdsmouth (shortened 0.75″ for half the ridge).
- Add 17.89″ of sloped length for the 16″ overhang.
- Per foot of run: common 13.42″, hip/valley 18.00″.
The geometry behind the cuts
A common rafter is just a right triangle. The run is half the building span; the rise is the run times the pitch (rise-per-12); and the rafter line length is the hypotenuse, √(run² + rise²). Because the triangle is fixed by the pitch, the plumb cut (at the ridge and tail) and the seat cut (the birdsmouth) are set with a framing square — the same pitch you'd read on the roof pitch converter.
Two shortcuts framers use: the rafter travels √(144 + pitch²) inches per foot of run (the "per-foot length"), and a hip or valley rafter travels √(288 + pitch²) per foot of common run because it runs diagonally. The rafter is shortened by half the ridge thickness, then the sloped overhang is added past the wall. Cutting the stringer for the stairs instead? See the stair stringer calculator.
This is layout geometry, not a structural design — rafter size, spacing, and allowable spans come from the adopted code's span tables (or an engineer). Verify against your AHJ.
Frequently asked questions
How do you calculate rafter length?
The common rafter is the hypotenuse of the run and rise: length = √(run² + rise²). The run is half the span; the rise is the run times the pitch over 12. A 24-foot span at 6:12 gives a 12-foot run, a 6-foot rise, and a rafter about 13.42 feet long (before the overhang).
What are the plumb cut and seat cut on a rafter?
The plumb cut is the vertical cut where the rafter meets the ridge (and the tail); the seat cut is the horizontal cut of the birdsmouth that sits on the wall plate. Both come from the pitch — set a framing square to the pitch (rise on the tongue, 12 on the body) to mark them.
What is the rafter length per foot of run?
A handy factor: √(144 + pitch²) inches of rafter for every 12 inches of run. At 6:12 that's about 13.42 inches per foot of run; for a hip or valley rafter it's √(288 + pitch²) — about 18 inches per foot of common run at 6:12.
Does the ridge board change the rafter length?
Yes — the rafter runs to the ridge centerline, so it's shortened by half the ridge board's thickness (measured along the run). Enter the ridge thickness and the tool subtracts it for you.