Rafter Length Calculator
Enter the horizontal run and roof pitch to find the rise and rafter length.
Pitch is expressed as rise per 12 inches of run — a 6/12 pitch means 6 inches of rise for every 12 inches of run.
How rafter length is calculated
Rise is found by multiplying the run by the pitch ratio (pitch divided by 12) — a 6/12 pitch over a 12-foot run gives a 6-foot rise. Rafter length is then found using the Pythagorean theorem: rafter length = √(run² + rise²), treating the run, rise, and rafter as a right triangle.
This gives the theoretical rafter length along the roof's slope. Actual cut rafters typically need additional length added for the overhang (eave) beyond the wall, and the exact cut angles depend on the specific rafter type and ridge configuration.
Frequently asked questions
What does a 6/12 roof pitch mean?
It means the roof rises 6 inches vertically for every 12 inches of horizontal run — a moderate-slope roof. A steeper roof might be 9/12 or 12/12, while a low-slope roof could be 3/12 or 4/12.
Does this account for the roof overhang?
No — this calculates the rafter length along the main roof slope from the wall plate to the ridge. Add extra length separately for any eave overhang beyond the exterior wall.
Is the run the full building width or half of it?
For a simple gable roof, run is typically half the building's width (from the exterior wall to the center ridge line), since the roof slopes down symmetrically on both sides from that peak.