How it's calculated
rafter = (run + overhang) × √(1 + (pitch ÷ 12)²)
run = horizontal half-span, overhang = eave length, pitch = rise per 12 units of run. The √ term is the slope multiplier; common rafters use ×1, hip/valley rafters use a larger multiplier.
Common questions
How does pitch change rafter length?
A steeper pitch makes the rafter longer for the same run, because the rafter is the hypotenuse of the run-and-rise triangle.
What is the rafter run?
The run is the horizontal distance from the wall to the ridge, measured level, not along the slope.