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Roof Pitch Calculator
Roof pitch describes steepness as the rise over a 12-inch run. Enter any rise and run — measured in the same unit — to get the pitch in x/12 form, the angle in degrees, the slope as a percent, and the rafter length factor used to size rafters and estimate roofing area.
Results update as you type. Estimates only — confirm final quantities before ordering.
The formula
Angle = arctan(Rise ÷ Run)
Rafter length factor = √(1 + (Rise ÷ Run)²)
The rafter factor converts a horizontal run into the actual sloped length: multiply any horizontal distance by the factor to get the length along the roof. The same factor converts a flat (plan) roof area into the true sloped surface area you order shingles for.
Worked example
Example: a roof that rises 6 inches for every 12 inches of run.
- Pitch: (6 ÷ 12) × 12 = 6 / 12
- Angle: arctan(0.5) = 26.57° | Slope: 50.0%
- Rafter factor: √(1 + 0.25) = 1.118 — so a 20 ft horizontal run needs a 20 × 1.118 = 22.36 ft rafter
Reference chart
| Pitch | Angle | Rafter factor |
|---|---|---|
| 1 / 12 | 4.76° | 1.003 |
| 2 / 12 | 9.46° | 1.014 |
| 3 / 12 | 14.04° | 1.031 |
| 4 / 12 | 18.43° | 1.054 |
| 5 / 12 | 22.62° | 1.083 |
| 6 / 12 | 26.57° | 1.118 |
| 7 / 12 | 30.26° | 1.158 |
| 8 / 12 | 33.69° | 1.202 |
| 9 / 12 | 36.87° | 1.250 |
| 10 / 12 | 39.81° | 1.302 |
| 12 / 12 | 45.00° | 1.414 |
Frequently asked questions
What is the most common roof pitch for a house?
Most US homes fall between 4/12 and 6/12 — steep enough to shed water and snow reasonably, shallow enough to walk with care and keep shingle costs normal. Ranch homes trend lower (3/12–4/12); traditional and northern snow-country designs run 6/12 and up.
What is the difference between pitch and slope percent?
Pitch states rise per 12 units of run (6/12); slope percent states rise ÷ run × 100 (50% for the same roof). They are two labels for the same steepness — our Slope Calculator converts between percent, degrees and ratio.
What is the minimum pitch for asphalt shingles?
Standard practice and the IRC allow shingles down to 2/12 with special double underlayment between 2/12 and 4/12; 4/12 and above uses normal underlayment. Below 2/12, use a low-slope membrane system instead. Always confirm with the shingle manufacturer's instructions.
How do I measure pitch from inside the attic?
Hold a 12-inch level horizontally against the underside of a rafter and mark its end; measure straight down from that mark to the rafter. That vertical distance is the rise per 12 of run — the pitch numerator. No roof-walking required.
Related calculators
- Slope Calculator — Slope as a percent, an angle in degrees, and a ratio.
- Board Foot Calculator — Board feet and lumber cost from thickness, width, length and quantity.
- Drywall Calculator — Sheets of 4×8 and 4×12 drywall for your wall and ceiling area.
Sources & method: Standard roof-framing geometry (factor = √(1 + (rise/run)²)); pitch/angle/factor values match published rafter tables (NRCA roofing references); cross-checked against Omni Calculator — Roof Pitch Calculator.