How to calculate roof pitch in degrees, ratio and percent
Roof pitch is just a right-angled triangle: the rise, the run and the rafter.
In Australia roof pitch is usually given in degrees. Older plans and US sources may use "x in 12". All you need is the vertical rise and the horizontal run, which is half the span for a simple gable roof.
The formulas
| You want | Formula |
|---|---|
| Pitch in degrees | arctan(rise ÷ run) |
| Pitch as x in 12 | rise ÷ run × 12 |
| Slope % | rise ÷ run × 100 |
| Rafter length | √(rise² + run²), plus the overhang |
| Roof surface area | plan area × (rafter ÷ run) |
Worked example
A garage 4.8 m wide has a run of 2.4 m. With a 600 mm rise, the pitch is arctan(600 ÷ 2,400) = 14.0°, which is 3 in 12 or 25%. The rafter is √(600² + 2,400²) = 2,474 mm before the overhang. The roof area factor of 1.031 means the roof surface is about 3% bigger than its footprint.
Measuring an existing roof
Hold a level horizontally against a rafter, measure 1,000 mm along it, then measure straight up to the underside of the rafter. That gives you the rise per 1,000 mm of run.
Many phone level apps show the angle when you lay the phone on a rafter or the edge of the barge board.
✨ Key Takeaways
- Pitch = arctan(rise ÷ run).
- Rafter length = √(rise² + run²), plus the overhang.
- Check the material's minimum pitch first.
Open the roof pitch calculator →