Rafter Length Calculator
Calculate common rafter line length, overhang, total length, ridge height, and cut angles. Enter your building span and pitch — or rise and run directly.
Line length vs. total rafter length
Line length is the rafter measurement from the center of the ridge to the outside of the wall plate — it does not include the overhang tail. Total rafter length adds the overhang. Most framing squares and rafter tables give line length, so always add your overhang separately.
Ridge board deduction
The run used to calculate rafter length is measured to the center of the ridge. Since the ridge board has thickness, you deduct half the ridge thickness from the run before calculating. For a standard 2× ridge board (1.5" thick), deduct 0.75" from the run.
Plumb cut angle
The plumb cut (also called the ridge cut or bird's mouth plumb cut) is set at the arctangent of rise over run. For a 6/12 pitch, that's 26.6°. Set your circular saw bevel to this angle for the plumb cut at the ridge and the heel cut at the bird's mouth.