What is a Rafter Length Calculator?
A rafter length calculator finds the length of a roof rafter — the sloped structural member that runs from the ridge to the wall plate. Because the rafter forms the hypotenuse of the right triangle made by the roof's vertical rise and horizontal run, its length follows directly from the Pythagorean theorem. This tool works in any consistent unit (inches, feet, metres) and also reports the roof pitch angle and the common "rise per 12" slope notation.
How to use it
Enter the rise (how far the roof goes up) and the run (the horizontal distance covered). Optionally add an overhang length to extend the rafter past the wall. The calculator returns the rafter length, the total length including overhang, the pitch angle in degrees, and the slope expressed as rise per 12 units of run.
The formula explained
The core equation is $$L = \sqrt{\text{rise}^{2} + \text{run}^{2}}$$ The pitch angle is $$\theta = \arctan\!\left(\frac{\text{rise}}{\text{run}}\right)$$ and the slope ratio is $$\frac{\text{rise}}{\text{run}} \times 12$$ giving the familiar "X-in-12" carpentry expression. The overhang is simply added on to the base rafter length to give the total board length you need to cut.
Worked example
For a roof with a rise of 5 and a run of 12: $$L = \sqrt{25 + 144} = \sqrt{169} = 13$$ The pitch angle is \(\arctan(5/12) \approx 22.62°\), and the slope is \((5/12)\times 12 = 5\) in 12. Add a 1-unit overhang and the total length becomes 14.
FAQ
What units should I use? Any unit, as long as rise and run use the same one — the result comes back in that same unit.
Does this include the overhang? The "Rafter Length" is the rise/run hypotenuse only; the "Total length" line adds your overhang on top.
What is roof pitch? Pitch describes steepness, often written as rise-in-12 (e.g. 5/12) or as an angle in degrees — both are shown here.
