Rafter Calculator
Enter the building span and the pitch — get the common rafter length including the tail, the plumb and seat cut angles, and (with the roof length) rafter counts, lumber footage, and the sloped roof area in squares for shingles and underlayment.
Planning tool — results are estimates for quoting and ordering, not engineering or building-code advice.
Common questions
How is rafter length calculated?
The horizontal run is half the span minus half the ridge thickness. Rafter line length is the hypotenuse of run and rise (rise = run × pitch ÷ 12), and the overhang tail is added at the same slope — the classic framing-square 'length per foot of run' method.
What are the plumb and seat cuts?
The plumb cut is the vertical-face angle used at the ridge and the tail end; the seat cut is the level cut of the birdsmouth that sits on the wall plate. They always add up to 90°.
How do I convert pitch to degrees?
Pitch is rise per 12 inches of run, so the angle is atan(rise ÷ 12). A 6/12 roof is 26.6°; the calculator shows the plumb cut, which is the same angle.
Related: Square-up calculator · Spacing calculator
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