Rafter Length Calculator (run, overhang & pitch)
Get the line length of a common rafter from the horizontal run, the overhang and the pitch. This is layout geometry — the length along the top of the rafter — not a structural member size.
Calculator
A 14.0 ft run with 1.0 ft overhang at 6/12 needs a 16.77 ft rafter (line length; this is geometry, not a structural size).
A common rafter follows the slope from the wall plate up to the ridge. Its line length — the measured distance along the top edge — is pure geometry: the horizontal run and the pitch define a right triangle, and the rafter is its hypotenuse. Add the overhang and you have the full length to lay out before cutting the plumb, seat and tail cuts.
Enter the run (the horizontal distance from the outside of the wall to the ridge — for a symmetric gable that is half the building span), the overhang (how far the eave projects horizontally past the wall), and the pitch. The tool multiplies the total horizontal distance by the pitch multiplier to give the sloped rafter length, and also shows the length without the overhang so you can locate the bird’s-mouth seat cut at the wall.
Important: this is a geometric length for layout and estimating stock, not a structural design. Choosing the rafter’s depth, thickness and spacing to carry snow, wind and dead loads is a job for span tables or a licensed engineer.
Formula
- Multiplier:
M = √(1 + (pitch/12)²) - Rafter length:
rafter = (run + overhang) × M - Without overhang:
(run) × M
The multiplier is the sloped length per unit of horizontal distance — the same number that scales footprint to roof area.
Worked example
A 14 ft run with a 1 ft overhang at 6/12:
- Multiplier:
√(1 + 0.5²) = 1.1180 - Full length:
(14 + 1) × 1.1180 = 16.77 ft - Wall to ridge only:
14 × 1.1180 = 15.65 ft
So each common rafter is 16.77 ft of stock, with the seat cut set 15.65 ft up the line from the ridge measurement point.
Line length, cuts & structure
Line length vs. board length. The result is the theoretical line length along the top edge of the rafter, measured to the centerline of the ridge. In practice you deduct half the ridge board thickness at the top and account for the plumb and tail cuts at the ends. Framers lay out from the line length and then adjust for these real-world cuts.
Overhang is horizontal. Enter the overhang as its horizontal projection past the wall, not the sloped distance — the multiplier converts the whole horizontal figure to slope in one step. If your plans give the sloped tail length instead, leave the overhang at zero and add it to the result directly.
Related layout. Pair this with the ridge height calculator to set how high the ridge sits, and the pitch calculator to confirm the slope. For a metal roof, the rafter length is also the panel length per slope.