A roof pitch calculator converts between the ways a roof slope is described. Enter the rise and run or a roof angle, and the tool returns the pitch as an x-in-12 ratio, in degrees, and as a percentage slope — useful for ordering materials, checking a drawing, or matching an existing roof.
How it works
The slope is the rise ÷ run ratio. From rise and run the tool computes:
- x-in-12 ratio = rise ÷ run × 12 (vertical rise per 12 of horizontal run)
- Angle = arctan(rise ÷ run), in degrees
- Percentage slope = rise ÷ run × 100
In angle mode it works backward: the slope is tan(angle), so rise-in-12 = tan(angle) × 12 and percentage = tan(angle) × 100. All three formats describe the same roof slope.
Conversion table
A roof with a rise of 4 over a run of 12 (a 4:12 roof):
- Ratio = 4 in 12
- Angle = arctan(4 ÷ 12) ≈ 18.43°
- Percentage = 4 ÷ 12 × 100 ≈ 33.3%
| x-in-12 | Angle | % slope | Common use |
|---|---|---|---|
| 1:12 | 4.8° | 8.3% | Very low slope, flat-look membrane |
| 3:12 | 14.0° | 25% | Low slope, needs ice shield |
| 4:12 | 18.4° | 33.3% | Common residential |
| 6:12 | 26.6° | 50% | Steep residential |
| 9:12 | 36.9° | 75% | Very steep, harder to walk |
| 12:12 | 45.0° | 100% | Equal rise and run |
What pitch is right for different roofing materials?
Not every roofing product can be applied to every slope. Manufacturers publish minimum pitch requirements:
- Asphalt shingles — typically require a minimum of 2:12 to 4:12 depending on the product; most standard shingles require at least 4:12
- Metal standing seam — can go as low as 1:12 to 3:12 depending on the panel system and seam height
- Clay and concrete tiles — generally require 4:12 or steeper; some profiles need 5:12 or more for adequate drainage
- Wood shakes — typically 4:12 minimum; prone to moisture issues on low slopes
- EPDM and TPO membrane — suitable for very low slopes approaching flat (under 2:12)
Always verify the manufacturer’s minimum pitch specification before ordering materials.
Measuring an existing roof
If you are trying to find the pitch of an existing roof without blueprints, a pitch gauge tool (a spirit level with a built-in angle readout) is the most direct approach. Alternatively, place a 12-inch level horizontally against the rafter and measure vertically from the end of the level down to the rafter surface — that measurement in inches is the rise in a 12:12 run, giving you the x-in-12 pitch directly.
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