How to use the roof pitch calculator
Pick a mode at the top of the calculator. In pitch to results mode, enter a rise and a run and every other value, angle, slope factor, grade, and the equivalent x/12 pitch, updates immediately. In angle to pitch mode, enter an angle in degrees instead and the calculator solves backward for the rise per 12 inches of run, along with slope factor and grade. Both modes share the same live diagram underneath, so you can see the triangle change shape as you type, not just read a new number.
Unlike a printed pitch chart, which only lists whole-number rises from 1/12 through 24/12, this calculator accepts any value. Type 5.5, 7.25, or any decimal rise and it solves exactly, with no need to round to the nearest table row. The run field is editable too, so a measurement that was not taken over exactly 12 inches, for example a 9 inch rise over a 17 inch run, still gets an exact answer instead of an approximation.
Pitch, angle, slope factor, and grade: what each one is for
Pitch (rise over run, written as x/12) is the number roofers, material spec sheets, and permits use in everyday conversation, because it maps directly to how a roof is physically framed. Angle in degrees is what a saw, a speed square, or a framing calculation needs when the input has to be a degree setting rather than a ratio. Slope factor is the multiplier that converts a flat footprint area into the true sloped roof area, the number that actually drives how much material a job needs. Grade, the same ratio expressed as a percentage instead of a fraction, shows up mostly in drainage and civil contexts and on some manufacturer minimum-slope specifications. All four describe the same slope; which one you reach for depends on which document or tool you are talking to.
Why non-integer pitches happen
A pitch chart assumes clean, whole-number rises because that is how lumber, rafter tables, and most reference material are organized for convenience. Real roofs do not always land on a round number. A house that has settled slightly over decades can have a roof plane that reads a few tenths of an inch off what it was framed at. An addition tied into an existing roofline often has to match whatever angle the original roof happens to be, which is rarely a clean integer. Custom or architect-designed roofs sometimes specify an exact angle in degrees rather than a pitch at all. In every one of these cases the trigonometry does not care whether the rise is a whole number, so a calculator that accepts decimals gives an exact answer instead of forcing a rounding error onto the job.
Worked examples
A rise of 5.5 over a run of 12 works out to a 24.62 degree angle, a 1.100 slope factor, and a 45.83 percent grade, a fraction between the common 5/12 and 6/12 pitches on a standard chart. Working the other direction, an angle of 30 degrees solves back to a rise of about 6.93 over 12, a 1.155 slope factor, and a 57.74 percent grade, right between a 6/12 and 7/12 pitch. Feeding that resulting rise and run back through the forward formulas returns 30.00 degrees exactly, confirming the two directions agree.
Measuring on site for this calculator
Hold a level horizontal against the underside of a rafter or the roof deck, extend it out a known distance, and measure straight down to the roof surface at that mark with a tape measure. That vertical distance is your rise, and the horizontal distance you measured out is your run: enter both here exactly as measured, rather than rounding either one to match a chart. If you only have an angle reading from a digital angle finder or a speed square's degree scale, use angle to pitch mode instead and skip the rise and run measurement entirely.
For the full standard whole-number reference from 1/12 through 24/12, see the Roof Pitch Chart. A rafter length calculator for full board lengths, not just the per-run ratio shown here, is planned as a future addition to this hub.