Roof Pitch Calculator
Get your roof's pitch ratio, angle in degrees, and slope percentage from rise and run — plus rafter length and roof area.
How to use the roof pitch calculator
- 1
Measure your roof's rise and run — 12 inches (or 30 cm) is a standard run reference.
- 2
Your pitch ratio, angle, and slope percentage appear instantly.
- 3
Check rafter length & roof area and enter the eave-to-ridge length and ridge width.
- 4
Optionally add a rafter overhang — your rafter length and true roof surface area update instantly.
Understanding rise, run, and roof pitch
Run is a horizontal distance — measured along a perfectly level line, never along the slope itself. Rise is the vertical height gained over that same horizontal distance. Roofers traditionally measure both over a 12-inch run, which is why pitch is written as a ratio like "6/12" rather than a simplified fraction.
From rise and run, the calculator derives everything else:
- Pitch ratio = rise ÷ run, normalized to a 12-unit run (so it always reads as "X/12", the standard roofing convention)
- Pitch percentage = (rise ÷ run) × 100
- Angle = atan(rise ÷ run), converted to degrees
- Slope factor = √(rise² + run²) ÷ run — the multiplier that converts a flat, horizontal length into its actual sloped length
Worked example: A roof with a rise of 6 inches over a 12-inch run has a pitch of 6/12, a pitch percentage of 50%, and an angle of atan(6 ÷ 12) ≈ 26.6°. Its slope factor is √(6² + 12²) ÷ 12 ≈ 1.118 — so a rafter that spans 15 feet of horizontal run is actually 15 × 1.118 ≈ 16.77 feet long, and a roof plane 15 ft (horizontal) × 30 ft covers 16.77 × 30 ≈ 503 sq ft of actual surface, not the 450 sq ft the flat footprint suggests.
Quick pitch reference table
| Pitch | Angle | Slope % | Typical use |
|---|---|---|---|
| 3/12 | 14.0° | 25% | Minimum slope for standard shingles |
| 4/12 | 18.4° | 33.3% | Common on ranch-style homes |
| 5/12 | 22.6° | 41.7% | Traditional residential slope |
| 6/12 | 26.6° | 50% | Most common residential pitch |
| 7/12 | 30.3° | 58.3% | Moderately steep |
| 8/12 | 33.7° | 66.7% | Steep, common on Craftsman style |
| 9/12 | 36.9° | 75% | Steep |
| 10/12 | 39.8° | 83.3% | Very steep, typically needs fall protection |
| 12/12 | 45.0° | 100% | Extremely steep, a true 45° angle |
Typical-use notes are general roofing conventions, not building code — check local code and manufacturer specifications for any given roofing material.
Tips for measuring rise and run safely
- Measure from the attic, not the roof. Hold a torpedo level flat against the underside of a rafter, extend it until it reads perfectly level for exactly 12 inches, then measure straight down from that 12-inch mark to the top of the rafter. That vertical measurement is your rise.
- Never estimate pitch by eye from the ground. Roofs consistently look shallower than they are from a distance — always take an actual measurement.
- If you must measure outside, stay off the roof itself. Use a level held against a gable end or fascia board where you can stand safely on a ladder, ground, or scaffold.
- Double-check with a second measurement at a different point along the same roof plane — pitch should be identical anywhere along a straight roof plane, so a mismatch means a measurement error.
- Leave steep roofs (over roughly 6/12) to professionals with proper fall-protection equipment rather than measuring from the roof surface directly.
Pouring footings or a foundation for the same project? Try the Concrete, Mulch & Topsoil Calculator →
Frequently asked questions
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