Home & build
Calculate roof pitch, angle, and rise from run and ridge height.
Enter your numbers to see the answer.
Roof pitch calculator guide
Roof pitch is written in the building trades as inches of rise per 12 inches of horizontal run, so a 6/12 roof climbs 6 in for every foot it travels toward the ridge. This calculator takes any rise and run you measure, normalises them to that x/12 form, and also gives the slope as an angle in degrees, which is what you need for a mitre saw, a CAD drawing, or a solar panel tilt. You can measure the rise over a 12 in run directly with a level and tape on the underside of a rafter, or enter the full ridge height and half the building span. Pitch determines which roofing materials are allowed, how underlayment is installed, how rafters are cut, and how much the roof surface area exceeds the footprint. The result is geometric only; check the material manufacturer’s minimum slope and local code before choosing a covering.
Pitch (x/12) = rise ÷ run × 12. Angle (°) = arctan(rise ÷ run). Slope (%) = rise ÷ run × 100. Rafter length per foot of run = √(12² + pitch²) in.
Worked example: a rafter rises 6 in over a 12 in run. The pitch is 6 ÷ 12 × 12 = 6, written as 6/12 or “6 in 12”. The angle is arctan(6 ÷ 12) = arctan(0.5) = 26.57°, which the calculator rounds to 26.6°. The slope is 50%. For every 12 in of horizontal run the rafter itself is √(12² + 6²) = √180 = 13.42 in long, so a roof with a 12 ft half-span has a rafter line of 12 × 13.42 = 161 in, about 13.4 ft. The calculator displays a 6/12 pitch and a 26.6° angle.
Rise and run take the same unit, inches by default, but any consistent unit works because the tool uses their ratio; a ridge height of 4 ft over a 12 ft half-span can be entered as 48 and 144. Pitch is shown as inches per 12 in of run, and the angle in degrees to one decimal. Metric roofs are usually quoted in degrees or as a ratio such as 1:2, which is the same as 6/12.
The roof pitch calculator returns a quantity to buy, not a guarantee of coverage. Substrate condition, waste from cuts, spillage, and settling all push real consumption above the theoretical figure, which is why most trades order a margin on top. Check the coverage rate printed on the product you actually purchase, because it varies between brands and mixes. On this page the figure rests entirely on rise and run, so start there if the roof pitch calculator returns something you did not expect.
Good to know: run is the horizontal distance, not the sloped rafter length, so measuring along the rafter with a tape gives a pitch that is too low. Use half the building width for run when you enter the ridge height, and measure rise from the top of the wall plate, not the ground. The rise is 6 in per 12 in only on the rafter’s underside or a straight-edge; shingle thickness and gutters distort a reading taken on the surface. Pitch is not the same as a 6° or 6% slope.
Construction quantities are planning estimates. Confirm product yield, waste, site conditions, structural requirements, and local code with the supplier or qualified professional.
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How it works
Calculate roof pitch, angle, and rise from run and ridge height. This tool explains the calculation so you can adjust the assumptions to match your situation.
Worked example
With Rise = 6 in · Run = 12 in → 6/12 pitch (26.6° roof angle). Change an input above and this example updates with your numbers.
Common questions
A 6/12 pitch rises 6 inches per 12 inches of run, which is arctan(0.5) = 26.57°, or about 26.6°. For comparison, 4/12 is 18.4°, 8/12 is 33.7°, and 12/12 is 45°.
Hold a level horizontally against the underside of a rafter or the roof surface, mark 12 inches along the level, and measure straight up from that mark to the roof. That vertical distance in inches is the pitch. Alternatively divide the ridge height by half the span and multiply by 12.
Most asphalt shingle manufacturers and the International Residential Code allow shingles down to 2/12, but slopes between 2/12 and 4/12 require special double-layer or self-adhering underlayment. Below 2/12, use a membrane or metal roof designed for low slopes.
The roof surface is larger than the footprint by a factor of √(12² + pitch²) ÷ 12. At 6/12 that factor is 1.118, so a 1,500 sq ft footprint has about 1,677 sq ft of roof. At 12/12 the factor is 1.414.
A 30° slope rises 12 × tan(30°) = 6.93 inches per foot of run, so it is just under a 7/12 pitch. Enter a rise of 6.93 and a run of 12 to see it confirmed at 30.0°.