How to use the roof pitch calculator
Choose the slope value you know: vertical rise and horizontal run, rise per 12 inches of run, an angle above horizontal, or slope percentage. Enter the value and read the other formats. If you also know the horizontal distance from wall plate to ridge centerline, enter it to see a theoretical straight-line rafter length.
Formulas and result meaning
- Ratio: rise ÷ horizontal run.
- x:12 pitch: ratio × 12. The large pitch label rounds this to the nearest quarter inch; a calculated value rounded to two decimals is also shown.
- Angle: arctan(ratio) × 180 ÷ π, in degrees.
- Slope percentage: ratio × 100.
- Slope factor: √(1 + ratio²). For one simple roof plane, multiply its horizontal projected area by this factor to estimate its sloped area.
- Optional theoretical rafter line: horizontal run length × slope factor. This is the straight line from wall plate to ridge centerline. It excludes overhang, ridge thickness, birdsmouth and other cuts, framing details, and structural design.
Rise and run may both use inches or both use centimeters; their ratio is unitless. Optional line lengths can use feet or meters. The NIST SI conversion table defines 1 inch as 2.54 cm and 1 international foot as 0.3048 m.
Checked examples
Rise 6 in, run 12 in: ratio 0.5, pitch 6:12, slope 50%, angle 26.57°, and factor 1.1180. With a 12 ft horizontal run, the theoretical line is 12 × √1.25 = 13.42 ft.
Angle 30°: tan(30°) ≈ 0.57735. Calculated pitch is 6.93:12, displayed as approximately 7:12 after quarter-inch rounding. With a 14 ft horizontal run, the theoretical line is 14 ÷ cos(30°) ≈ 16.17 ft.
Limits and safe use
In U.S. practice, the 2024 International Residential Code roof assemblies chapter expresses slopes as units vertical per 12 units horizontal. It specifies different minimum slopes and installation requirements by roof covering; for example, its asphalt-shingle provisions use 2:12 and 4:12 thresholds. A 0:12 result is valid geometry, but roofing systems may require positive minimum slope. The adopted local code and manufacturer instructions can differ. This calculator does not determine code compliance.
The calculator models one constant slope. It does not account for multiple roof planes, irregular geometry, openings, overhangs, roof assemblies, structural loads, or framing cuts. A pitch number alone cannot determine which roofing product is suitable. Check the current instructions for the exact material and local project requirements. For example, GAF publishes product-specific low-slope installation instructions.
Measure from a safe accessible location; do not climb a roof solely to use this calculator. OSHA’s residential fall-protection guidance explains why roofing work needs a separate safety plan. Sources reviewed 29 September 2026.
Continue your estimate
Once you know the roof pitch, use the Rafter & Birdsmouth Calculator to convert that slope into common-rafter length and cut geometry.
Use the Roof Area Calculator for projected dimensions and the Roofing Material Calculator for a material estimate.
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