Roof Pitch Calculator — Rise/Run, Degrees, Slope & Rafter Length

Measure the vertical rise and the horizontal run using the same unit, then divide rise by run and multiply by 12. That gives the X/12 pitch. Example: 6 inches of rise over 12 inches of run is 6/12. The angle is arctan(rise/run), the percent slope is rise/run x 100, and the pitch multiplier is the square root of 1 plus the slope ratio squared. The math is simple. The measurement is where people usually mess it up.

A 6/12 roof rises 6 inches for every 12 inches of horizontal run. In metric, same idea: 150 mm rise over 300 mm run is still 6/12 because the ratio is the same. A 6/12 pitch is about 26.57 degrees, 50% slope, and has a pitch multiplier of 1.118. It is a common residential pitch because it sheds rain well without turning every repair into a rope-and-harness job.

Take the pitch number, divide it by 12, then use arctan. For a 4/12 roof, 4 divided by 12 is 0.333. Arctan(0.333) is 18.43 degrees. For 6/12, arctan(0.5) is 26.57 degrees. For 12/12, arctan(1) is 45 degrees. If you are reading plans, do not swap degrees and pitch ratio. A 30-degree roof is not 30/12. It is about 6.93/12.

Pitch multiplier tells you how much longer the sloped roof surface is compared with the flat plan view. It equals 1/cos(angle), or square root of 1 plus rise/run squared. Multiply flat roof footprint area by this number to estimate actual roof surface. A 1,500 sq ft gable footprint at 8/12 uses a 1.202 multiplier, so the sloped surface is about 1,803 sq ft before waste, laps, hips, valleys, and overhangs.

Code and manufacturers often allow asphalt shingles down to 2/12 only with special low-slope underlayment. My working answer is 4/12 for a normal asphalt shingle roof. 3/12 is the lowest pitch I would even discuss, and only with the right product, perfect decking, and a contractor who will put the low-slope details in writing. 3:12 is the lowest pitch I'd put asphalt shingles on without losing sleep. Below that, use a membrane or a low-slope system.

Use the attic if you can get to the rafters safely. Hold a level against the underside of a rafter, mark 12 inches horizontally, and measure straight up to the rafter. That rise is your X/12 pitch. You can also use a smartphone inclinometer against the fascia, rake board, or underside of the roof framing. Take a few readings because old roofs sag, fascia boards are not always straight, and phone cases can throw off the reading.

Roof pitch is usually written as X/12: inches of rise per 12 inches of run. Slope can mean the same ratio, but on plans it often appears as degrees or percent. A 6/12 pitch is a 50% slope and a 26.57-degree angle. The words get mixed on job sites all the time. When ordering materials or checking code, say the full format: 6/12, 26.6 degrees, or 50%. Do not just say "six" and hope everyone heard the same thing.

Use half the building width as the run for one side of a simple gable roof, then multiply by the pitch multiplier. A 30-foot wide building has a 15-foot half-span. At 6/12, the multiplier is 1.118, so the rafter line length is 15 x 1.118 = 16.77 feet. That is line length, not the final cut length. Add overhang, account for birdsmouth depth, and subtract for ridge beam thickness according to the framing detail.

Snow is not simple. Roof shape, exposure, drifting, insulation, and local design loads matter. Still, a 7/12 or steeper roof sheds snow better than a shallow roof. Below 4/12, snow tends to sit, thaw, refreeze, and load the structure. In real snow country, do not design by pitch alone. Use the local building code snow load, check drift conditions at valleys and dormers, and have the structure sized by someone who does this for a living.

A 6/12 roof is walkable for some trained roofers in dry conditions, but that does not make it safe for everyone. Dust, dew, algae, loose granules, and cheap shoes change the answer fast. In the US, OSHA fall-protection rules kick in for construction work 6 feet or more above a lower level. If you are not trained, do not climb it just to prove a point. Use attic measurement, a ladder at the eave, or hire someone with the right fall protection.