Rafter Calculator: Length, Pitch & Cut Angles

A common rafter is the hypotenuse of a right triangle. The horizontal leg is the run — the distance from the outside of the wall to the centre of the ridge, which for a symmetrical gable is half the building's width. The vertical leg is the rise — how far the ridge sits above the top of the wall. The rafter itself spans from wall to ridge, so its line length is simply the square root of the run squared plus the rise squared.

Roofers describe steepness as pitch, written as inches of rise for every 12 inches of run. Divide the rise by the run and multiply by 12 and you have it: a 6 ft rise over a 12 ft run is a 6-in-12 roof. That same ratio is an angle — the arctangent of rise over run — which this tool reports in degrees for anyone setting a saw or a bevel. The plumb cut, the vertical cut where the rafter meets the ridge, is scribed at that slope angle off the level line; the seat cut, the level cut that sits on the wall plate, is its complement, the two always adding to 90 degrees.

This is pure layout geometry — line length and angles. It does not add tail length for an overhang, subtract half the ridge thickness, or account for the height-above-plate seat notch, and it is not a span check. Add your overhang, take off the ridge, and confirm member sizes and spans against your local code or a framing table.