Free planning calculator
Roof pitch calculator: slope, angle, and rafter line
Convert a rise-over-run roof pitch into degrees and a slope factor, and see the sloped rafter length across half a span.
Direct answer
Roof pitch is rise divided by run over a fixed run, usually 12. The angle from horizontal is arctan(rise / run), and the slope factor sqrt(1 + (rise / run)^2) converts any horizontal distance into its sloped length.
Interactive tool
Enter your measurements
Formula
Worked example
A 6-in-12 pitch is a ratio of 0.5. The angle is arctan(0.5) = 26.565 degrees and the slope factor is sqrt(1.25) = 1.118. Across a 28 ft span, one side runs 14 ft horizontally, so the sloped rafter line is 14 x 1.118 = 15.653 ft before overhang, ridge thickness, birdsmouth, and bearing detail are added.
Assumptions
- Both roof planes share one symmetrical pitch about a central ridge.
- Rise and run are entered in the same unit.
- The span is the horizontal distance across the building, not a distance along the slope.
Caveats
- The result is a geometric line length, not a cut list or a rafter size.
- Minimum pitch depends on the roof covering and its manufacturer instructions.
- Snow, wind, seismic, and dead loads determine member sizes and require a qualified designer.
Primary references
- 2024 International Residential Code: Building PlanningInternational Code Council; accessed 2026-08-02.
- Wood Handbook: Wood as an Engineering MaterialUSDA Forest Service, Forest Products Laboratory; accessed 2026-08-02.
Understand and continue
Roof pitch: choosing a slope and living with the consequences
How rise over run converts to degrees and a slope factor, and what changes across covering, area, cost, and usable space when the pitch changes.
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