Rafter Length Calculator
Calculate the length of a common rafter using your building width, roof pitch, and overhang — includes total run and rise.
Rafter length
15.09ft
In inches
181.1in
Total run
162in
This is the theoretical rafter length along the top edge — it doesn't subtract the ridge board thickness or account for birdsmouth/tail cuts. Have a carpenter square check your actual cut lines before cutting stock.
How the rafter length calculator works
A common rafter forms the hypotenuse of a right triangle, where the run is the horizontal distance it spans and the rise is how much it climbs over that distance. The Pythagorean theorem solves for the rafter's actual length along that diagonal.
Formula: run (in) = (building width ÷ 2 × 12) + overhang. Rise (in) = run × (pitch ÷ 12). Rafter length = √(run² + rise²).
Pair this with the Roof Pitch Calculator if you only know your roof's rise and run and need the pitch ratio first.
Frequently asked questions
Run is the horizontal distance from the outside of the exterior wall to a point directly below the roof's peak. For a standard gable roof, that's half the building's total width, plus however much overhang you're adding past the wall.
The overhang extends the run before the roof reaches its peak, so it lengthens the rafter along with everything else — a bigger overhang means a longer rafter, not just a longer eave.
No — this gives the theoretical rafter length along the top edge using the Pythagorean theorem. If you're using a structural ridge beam, subtract half its thickness from the result. Birdsmouth notches and tail cuts are a layout step that happens after you have this base length.
Roof Pitch gives you the angle, percentage grade, and area slope factor for material estimating. This calculator uses the same rise-and-run relationship to solve for an actual rafter length in feet and inches — useful when you're ready to cut framing lumber.