Roof Pitch and Slope Calculator

It means the roof climbs 6 inches vertically for every 12 inches of horizontal travel. In this tool that reads as a 6:12 pitch, an angle of 26.57 degrees, a slope factor of 1.118 and a 50 percent grade.

It takes the arctangent of rise divided by run and converts to degrees. For a 6:12 roof that is arctan(6/12), which rounds to 26.57 degrees. A 12:12 roof comes out at exactly 45 degrees.

The slope factor is the rafter length per unit of horizontal run, equal to the square root of rise squared plus run squared, divided by the run. Multiply your run by it to get rafter length or true sloped area for ordering shingles and underlayment.

No. The run field accepts any positive number, and the tool normalizes the result back to a run of 12 so the ratio is comparable. A rise of 5 over a run of 10, for example, still reads as a 6:12 pitch.

It multiplies the run you enter in feet by the slope factor for your pitch. A 20 foot run on a 6:12 roof gives 22.3607 feet, before you add any overhang or subtract for the ridge board.

The angle and grade are rounded to two decimals, the slope factor to four decimals and the rafter length to four decimals. That is plenty for framing and material takeoffs, though your saw cuts will round to the nearest practical mark.

Pitches below 4:12 are considered low slope and shed water slowly, so they often need a fully adhered membrane rather than standard shingles. Pitches of 9:12 and steeper are hard to walk and usually call for roof jacks or a harness.

Hold a level against a rafter or the underside of the roof in the attic, mark 12 inches along it while it reads level, then measure straight down to the roof at that mark. The vertical drop is your rise over a 12 inch run.

Yes. Every calculation runs in your browser with plain arithmetic, nothing is uploaded, and the tool keeps working with no connection once the page has loaded.