Roof Pitch Calculator
Pitch, angle, and rafter factor from rise over run. Click any i for detail.
Rise and run
Rise
i
Rise
Vertical rise over the run you measured. Same unit as run. |
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|---|---|
Run
i
Run
Horizontal run (often 12 for “X-in-12” pitch). |
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Building span (optional)
i
Span
Overall building width. Rafter length uses half-span × pitch factor. |
How to use this calculator
- Measure rise over a known run (or use 12).
- Read X-in-12 pitch, degrees, and slope %.
- Optional: enter span for a rafter-length sketch.
Results explained
A 6-in-12 roof rises 6 inches for every 12 inches of run (~26.6°). Rafter factor is √(1 + (rise/run)²).
Quick reference: pitch
Common roof pitches.
| Item | Detail |
|---|---|
| 3/12 | Low slope ≈ 14° |
| 6/12 | Common ≈ 26.6° |
| 9/12 | Steep ≈ 36.9° |
| 12/12 | 45° |
Birdsmouth, overhang, and ridge thickness are not in the rafter length.
How the estimate is built
pitch₁₂ = (rise/run)×12. degrees = atan(rise/run). rafter ≈ (span/2) × √(1+(rise/run)²).
Example scenario
6 rise over 12 run is 6:12 (26.6°). On a 24 ft span, each common rafter is about 13.42 ft before overhang.
FAQ
Pitch vs slope %?
Pitch is X-in-12. Slope % is rise/run × 100. Same triangle.
Hip/valley rafters?
Longer than commons. This page is common-rafter geometry.
Can I walk a 12/12?
It’s 45° — plan fall protection. Geometry ≠ safety plan.