Updated · Built and checked by the PlinzoTools team, formulas tested against published sources
Convert roof rise and run into pitch ratio, angle in degrees, and slope factor.
Roof pitch is conventionally expressed as inches of rise per 12 inches of run (like "6/12"), but that's not the only way to describe a roof's steepness -- this calculator also converts it into a real angle in degrees and the slope factor used to scale flat area into sloped roof area.
Enter the rise and run in inches (run defaults to the standard 12). The result shows the pitch ratio, the equivalent angle in degrees, and the slope factor.
Pitch Ratio = Rise ÷ Run
Angle (degrees) = arctan(Rise ÷ Run) × (180 ÷ π)
Slope Factor = √(Rise² + Run²) ÷ Run
Use this to understand a roof pitch quoted as a ratio (common in the US) in terms of an actual angle, or to get the slope factor needed for the Roofing Calculator's area conversion if you're calculating it manually.
The calculator only needs two numbers, but getting them is the hard part. The safest way to find your pitch is from the attic: hold a level horizontally against the underside of a rafter, mark 12 inches out along the level, then measure straight up from that mark to the rafter. That vertical distance is your rise over a 12-inch run. If the attic is inaccessible, plans or original blueprints often list the pitch. Climbing onto a roof just to measure it is rarely worth the risk, particularly on steeper slopes.
If you measure over a different run than 12 inches, that is fine. Enter the run you actually used. For example, a rise of 20 inches over a run of 40 inches gives the same 0.5 ratio as 6/12, and the tool will report the same angle and slope factor.
Suppose you measure a rise of 9 inches over a 12-inch run (hypothetical numbers). The ratio is 9 divided by 12, or 0.75. The angle is the arctangent of 0.75, which is about 36.87 degrees. The slope factor is the square root of (81 plus 144), which is 15, divided by 12, giving 1.25. That means every 1,000 square feet of flat footprint under the roof corresponds to roughly 1,250 square feet of actual roof surface. Compare that with a 6/12 roof at 1.118, and you can see how much extra material a steeper roof consumes.
Many roofing products list a minimum slope. Asphalt shingles, metal panels, and membrane roofing each have different low-slope limits set by their manufacturers and by local building codes. Use the pitch you calculate here to check the product's installation instructions and consult your local building department or a licensed roofer before committing to a material on a low-slope roof.
The roof rises 6 inches for every 12 inches (1 foot) of horizontal run -- a common way US roofing describes steepness without needing an angle measurement.
Pitches above about 9/12 (roughly 37 degrees) are generally considered steep and may need special safety precautions or equipment for roofing work.
Enter 4 as the rise and leave the run at 12. The calculator divides 4 by 12 and takes the arctangent, which comes out at roughly 18.43 degrees.
Yes. Enter any rise and run measured in the same unit. The ratio, angle, and slope factor depend only on their proportion, so 3 over 6 gives the same answer as 6 over 12.
No. The pitch ratio is rise divided by run. The slope factor is the length of the sloped surface divided by the horizontal run, so it is always greater than 1 for any pitched roof and is used to scale flat area into roof surface area.
An app can give a rough angle, and you can enter the result as a rise over 12 by using the tangent, but an attic measurement or the original plans are more reliable. Always prioritize safety over precision when working near a roof edge.