This roof pitch calculator finds your roof’s slope from its rise and run, then converts it to the standard x:12 pitch, the angle in degrees, and the rafter-length multiplier. Enter your measurements above to get all three instantly.
How to use this calculator
- Measure the rise — the vertical height the roof gains.
- Measure the run — the horizontal distance for that rise (usually 12 inches).
- Read your pitch (x:12), angle, and multiplier from the results.
How is roof pitch calculated?
Roof pitch is the ratio of vertical rise to horizontal run, expressed over 12 inches of run:
Pitch = Rise : Run (normalized to Rise : 12) Angle = arctan(Rise / Run) Rafter multiplier = √(Rise² + Run²) / Run
- Rise — vertical height (inches)
- Run — horizontal distance (inches)
- Multiplier — converts horizontal span to true rafter length
Example calculation
For a roof with a 6-inch rise over a 12-inch run:
- Pitch = 6:12
- Angle = arctan(6 / 12) = 26.57°
- Multiplier = √(6² + 12²) / 12 = 13.42 / 12 = 1.118
So a 20-foot horizontal span needs rafters about 20 × 1.118 = 22.4 feet long.
Common roof pitches
- Low slope: 2:12 to 4:12 — sheds and modern homes.
- Conventional: 4:12 to 9:12 — most residential roofs.
- Steep slope: 9:12 and above — needs extra fastening and safety gear.
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Frequently Asked Questions
What is a good roof pitch?
Most residential roofs fall between 4:12 and 9:12, which sheds water well and suits standard shingles. Lower slopes need membrane roofing; steeper slopes cost more to build.
How do I convert roof pitch to degrees?
Take the arctangent of rise divided by run. For a 6:12 pitch that is arctan(6/12) = 26.57 degrees, which the calculator computes for you.
What is the rafter multiplier?
It is the factor that turns a horizontal span into true rafter length: the square root of (rise squared plus run squared), divided by run.