Roof Rafter Length Calculator

About this calculator

A common rafter's length is found from the roof span and pitch: half the span gives the run, the pitch (often written as rise over a 12-inch run, like a 6/12 roof) gives the pitch angle via the arctangent of rise over run, and the rafter length itself equals the run divided by the cosine of that pitch angle — the hypotenuse of the right triangle formed by the run and the rise.

Builders, roofers and framing carpenters use this calculation to order the correct rafter stock length before cutting, to set the birdsmouth and plumb cuts at the right angle, and to check that a chosen pitch delivers enough headroom or the right roofline before construction begins, since getting the pitch angle wrong on paper means costly rework on the roof.

This calculator takes your roof span and pitch and returns the rafter length, the rise and the pitch angle, so you can order lumber and mark your cuts with the same numbers a framing crew would use.