Pythagoras builds rafters
A common rafter is the hypotenuse of a right triangle: half the building span is the run, and the pitch's rise per foot of run is the height. <strong>Rafter length = √(run² + rise²)</strong> — the calculator also reports the resulting pitch so you can confirm it matches the plans.
Add the overhang after
The calculated length runs from the wall plate to the ridge centerline. Rafter tails overhanging the eaves (usually 12–24 inches) add length beyond that, measured along the same slope. Birdsmouth cuts — the notch that seats the rafter on the plate — don't change the length but do limit how deep you can cut.