Rafter Length Calculator
Roof rafter length, angle, and rise from span and pitch
Frequently Asked Questions
Rafter length is the hypotenuse of a right triangle: the run (half the building span) and the rise (run × pitch ÷ 12) are the two legs. Rafter length = √(run² + rise²), then the overhang is added along the same slope angle.
Roof pitch is expressed as rise per 12 inches of run. A 6/12 pitch rises 6 inches for every 12 inches of horizontal run, which works out to a roof angle of about 26.6°.
No — this calculates the theoretical rafter length along the roof slope from the outer wall to the ridge centerline, plus your overhang. Actual lumber length needed is typically slightly longer to account for the ridge board thickness and birdsmouth (seat cut) at the wall — add a few extra inches when ordering material.
No. Always confirm rafter sizing (dimensional lumber size and spacing) against your local building code and span tables, especially for snow load areas — this calculator only computes geometry, not structural sizing.