How this roof pitch calculator works
Roof pitch describes how steep a roof is. It is the ratio of the rise (how far the roof climbs vertically) to the run (the horizontal distance it covers). Builders conventionally express pitch as a rise-in-12 figure — the number of inches the roof rises for every 12 inches of run — which is why you see pitches written like 4/12, 6/12 or 12/12.
From the rise and run, this tool computes three related views of the same slope. The angle in degrees is atan(rise / run). The percentage grade is rise / run × 100. The rafter multiplier is √(rise² + run²) / run — the length of sloped roof per unit of horizontal run — so multiplying it by a horizontal span gives the actual rafter length (before overhang). The conversion is two-way: enter an angle and the calculator backs out the pitch with rise = 12 · tan(angle).
atan(6 / 12) ≈ 26.57°, the grade is 6 / 12 × 100 = 50%, and the rafter multiplier is √(6² + 12²) / 12 ≈ 1.118 — so a 12-foot run needs a rafter of about 13.42 ft before overhang.Common roof pitches and their angles
| Pitch (rise in 12) | Slope angle | Grade |
|---|---|---|
| 3/12 | 14.04° | 25% |
| 4/12 | 18.43° | 33.3% |
| 6/12 | 26.57° | 50% |
| 9/12 | 36.87° | 75% |
| 12/12 | 45.00° | 100% |
Reference note: angles are rounded to two decimals. The rafter multiplier and rafter length describe the common (straight) rafter run only and exclude eave overhang, ridge-board thickness and birdsmouth cuts. Always confirm against local building codes.
Frequently asked questions
- What is roof pitch?
- Roof pitch is the steepness of a roof, expressed as the vertical rise over the horizontal run. By convention it is written as a rise-in-12 ratio — the inches the roof rises for every 12 inches of horizontal distance — so a 6/12 pitch rises 6 inches over each 12 inches of run.
- What does 6/12 mean on a roof?
- A 6/12 roof rises 6 units vertically for every 12 units of horizontal run. That equals a slope angle of about 26.57° and a grade of about 50%. The first number is the rise and the second is the run, which is almost always 12.
- How do I convert roof pitch to degrees?
- Take the arctangent of the rise divided by the run. For a rise-in-12 pitch the angle equals atan(rise / 12). For example a 6/12 pitch is atan(6 / 12) ≈ 26.57°, and a 12/12 pitch is atan(12 / 12) = 45°.
- What is a good roof pitch?
- It depends on climate and roofing material. Pitches from about 4/12 to 9/12 are common on houses and shed water well. Low slopes below 3/12 usually need special membranes, while steep pitches above 9/12 shed snow quickly but are harder to work on.
- How do I measure roof pitch?
- Hold a level horizontally against the slope, mark 12 inches along it, then measure straight down from that 12-inch mark to the roof. That vertical distance in inches is the rise, giving the rise-in-12 pitch directly. A level with a pitch gauge or a phone level app also works.
- How do I find rafter length from roof pitch?
- Multiply the horizontal run by the rafter multiplier — √(rise² + run²) / run. For a 6/12 pitch the multiplier is about 1.118, so a 12-foot run gives a rafter of about 13.42 feet, before adding any overhang.