Roof Pitch Calculator
Calculate roof pitch (slope), rise, run, and rafter length.
Convert between degrees, ratio, and percentage for any roof project.
Roof pitch is a measure of how steep a roof is. It is expressed as the ratio of vertical rise to horizontal run, typically written as “X in 12” in the United States — meaning the roof rises X inches for every 12 horizontal inches (1 foot). In metric countries, pitch is expressed as degrees or as a percentage grade.
The Rise-Over-Run Ratio
The most common format in North America: a 6/12 pitch means the roof rises 6 inches for every 12 inches of horizontal distance. A 12/12 pitch is a 45-degree angle. A 4/12 pitch is the minimum recommended for asphalt shingles.
Converting Between Formats
- Pitch to degrees: Angle = arctan(Rise ÷ Run)
- Degrees to pitch: Rise = Run × tan(Angle)
- Pitch to percentage: Percentage = (Rise ÷ Run) × 100
Rafter Length Formula
For a simple gable roof: Rafter length = √(Rise² + Run²) + overhang
This is just the Pythagorean theorem. The rafter is the hypotenuse of a right triangle.
Common Roof Pitches
| Pitch (X/12) | Degrees | Description |
|---|---|---|
| 2/12 | 9.5° | Very low. Asphalt only with doubled underlayment, otherwise metal or membrane |
| 4/12 | 18.4° | Low-slope minimum for asphalt shingles |
| 6/12 | 26.6° | Moderate slope, very common |
| 8/12 | 33.7° | Steep, good drainage, more attic space |
| 12/12 | 45° | Very steep, high attic, dramatic look |
Roofing Material Minimums
| Material | Minimum Pitch |
|---|---|
| Standing seam metal | 1/12 |
| Flat membrane: TPO (thermoplastic polyolefin) or EPDM (ethylene propylene diene monomer) | 0–¼/12 |
| Asphalt shingles | 2/12 (with underlayment) or 4/12 (standard) |
| Wood shakes | 4/12 |
| Clay/concrete tile | 4/12 |
| Slate | 4/12 |
Multiplier for Roof Area
Roof surface area is larger than the horizontal projection area. The multiplier is: Area multiplier = √((Rise/Run)² + 1)
Three ways to enter the pitch. Use the ratio when you know the rise per 12 (a 6/12 roof: enter 6). Use degrees when you have measured the angle directly. Use the third option, total rise, when you have neither: measure how far the roof climbs over the run you entered, in the same unit as that run, and the calculator works out the ratio for you. A roof that rises 10 ft across a 20 ft run comes back as 6/12.
For a 6/12 pitch, the multiplier is √(0.5² + 1) = 1.118 — so a 1,000 sq ft footprint has 1,118 sq ft of actual roof surface.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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