Roof Pitch Calculator
Roof pitch is written as rise-over-run — the inches a roof rises for every 12 inches it runs horizontally. Enter the rise to convert it to an angle in degrees, the slope factor used to size materials, and the rafter length for your span.
Common sizes
Jump to a pre-filled estimate:
How it works
The angle is the arctangent of rise ÷ 12. The slope factor is the rafter length per unit of run — √(rise² + 12²) ÷ 12 — and is what you multiply a footprint by to get true roof area. Rafter length is half the building span (plus overhang) times the slope factor.
The formula
Angle = atan(rise ÷ 12). Slope factor = √(rise² + 12²) ÷ 12. Rafter = (span ÷ 2 + overhang) × slope factor.
Worked examples
| Example | Result |
|---|---|
| 6:12 pitch, 30 ft span | The most common residential pitch: about 26.6°, slope factor 1.118, ~17.9 ft rafters including a 12-inch overhang. |
| 4:12 pitch (low slope) | A gentle slope of about 18.4°. |
| 12:12 pitch (steep) | A dramatic 45° roof, slope factor 1.414. |
Frequently asked questions
What does 6:12 roof pitch mean?
It means the roof rises 6 inches for every 12 inches of horizontal run — an angle of about 26.6°. The first number is the rise; the second is always the 12-inch run.
How do I convert roof pitch to degrees?
Take the arctangent of the rise divided by 12. For example, atan(6 ÷ 12) = 26.6°. This calculator does it instantly for any rise.
What is the slope factor used for?
Multiply a roof's flat footprint area by the slope factor to get the true, larger surface area you actually need to cover with shingles or metal.
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Method reviewed by The editorial team. See our methodology for sources and assumptions.