Roof Pitch Calculator
Enter a roof's rise and run to get the pitch as x-in-12, the slope angle in degrees, and the rafter length per unit of run.
How it works
Roof pitch is written as rise over run, standardized to a run of 12. If a roof climbs 6 inches for every 12 inches it runs horizontally, that's a 6:12 pitch — the way roofers and building plans describe slope.
The angle in degrees comes from the same two numbers: the arctangent of rise divided by run. A 6:12 pitch works out to about 26.6 degrees, which is handy when you're setting a saw or a rafter angle finder.
The rafter length is just the hypotenuse of that rise-and-run triangle — the square root of rise squared plus run squared. For a 6-and-12 triangle that's about 13.42, meaning the sloped rafter is 13.42 units for every 12 units of horizontal run.
Frequently asked questions
What's the difference between pitch and angle?
They describe the same slope two ways. Pitch is a ratio like 6:12 that carpenters use directly with framing squares, while degrees is the trigonometric angle. This tool shows both so you can use whichever your tools or plans call for.
How do I turn the rafter figure into a real length?
The rafter result is per unit of run — 13.42 per 12. Multiply it by your actual horizontal run in the same units to get the true rafter length, then add for overhang and adjust for the ridge board thickness.
What counts as a steep roof?
Anything up to about 3:12 is considered low-slope and needs special membrane roofing. Common shingle roofs sit around 4:12 to 8:12, and once you pass roughly 9:12 or 12:12 you're into steep territory that's hard to walk on.