Rafter Length Calculator

Calculate the exact length of a rafter from the horizontal run and roof pitch.

Ad Slot 1

Horizontal run (feet) — half the building widthRoof pitch (rise in inches per 12" of run)

Ad Slot 2

Rafter length formula

Rafter length is the hypotenuse of a right triangle where one side is the horizontal run and the other is the total rise. Using the Pythagorean theorem: Rafter = √(run² + rise²). The total rise = run × (pitch / 12). For example, a 12 ft run with a 6/12 pitch has a total rise of 6 ft, and a rafter length of √(144 + 36) = √180 = 13.42 ft.