How to calculate roof pitch
Roofers describe steepness three interchangeable ways, and this calculator converts between all of them. Pitch is written as rise-in-run, almost always normalized to a 12-inch run — so 4/12 means the roof climbs 4 inches for every 12 inches it travels horizontally. Angle is that same slope in degrees, found with arctan(rise ÷ run): 4/12 works out to 18.43°. Percent slope is rise ÷ run × 100, so 4/12 is a 33.33% grade. Because the run is fixed at 12, converting a pitch to a decimal is just rise ÷ 12. The tool accepts whichever value you have — a tape-measure rise and run, an angle off a smartphone inclinometer, or a percent — and returns the other two plus the simplified ratio.
The same rise-and-run numbers also give you rafter length and material eligibility. A common rafter is the hypotenuse of the rise-run triangle, √(rise² + run²), which the calculator expresses as a pitch multiplier you can multiply against any run: 1.054 at 4/12, 1.118 at 6/12, 1.414 at 12/12. Just as important, pitch decides what you are allowed to install. The International Residential Code sets a minimum slope for every covering — 1/4:12 for membranes and standing-seam metal, 2/12 for double-underlayment asphalt shingles, 3/12 for exposed metal and wood shingles, 4/12 for standard shingle application. Enter your slope and the calculator checks it against each R905 threshold so you find out before ordering material, not after a failed inspection.
Good to know
- Pitch is always rise over a 12-inch run — a '4 pitch' and '4/12' are the same thing; convert to degrees with arctan(rise ÷ 12).
- Below 2/12 you cannot use asphalt shingles at all; 2/12 to just under 4/12 requires a double layer of underlayment (IRC R905.2.2).
- Rafter length is √(rise² + run²), not the flat run — a 6/12 roof needs rafters about 12% longer than the horizontal span, before overhang.
- Roof-covering minimum slopes are code minimums, not recommendations; always confirm the manufacturer's own low-slope instructions and your local amendments.