Rafter Length Calculator
Enter your roof's horizontal run, pitch, and any eave overhang to calculate the exact rafter length needed, using the same rise/run relationship as the Roof Pitch Calculator.
How the Rafter Length formula works
The rafter forms the hypotenuse of a right triangle formed by rise and run:
Rise = Run × (Pitch / 12) Rafter length = √(Run² + Rise²) + Overhang
Step-by-step calculation
- Multiply the horizontal run by the pitch (rise per 12 inches of run) to find total rise.
- Apply the Pythagorean theorem to run and rise to find the sloped rafter length.
- Add any eave overhang length to get the total rafter length to cut.
Worked example
A run of 12 ft with a 6/12 pitch and a 1.5 ft overhang: Rise = 12 × (6/12) = 6 ft. Rafter length = √(12² + 6²) = √180 ≈ 13.42 ft. Total with overhang = 13.42 + 1.5 ≈ 14.92 ft.
Frequently asked questions
What's the difference between run and span?
Run is the horizontal distance from the wall plate to the roof's centerline (half the building width for a simple gable roof), while span is the full building width — this calculator uses run, which is typically half of span for a symmetrical roof.
Should I add extra length beyond this calculation for the actual lumber to cut?
Many builders add a small buffer beyond the calculated length to allow for the birdsmouth cut and ridge board fitting — this calculator gives the theoretical rafter length; actual lumber ordered is often slightly longer.