How to Calculate Roof Pitch
Measuring rise and run, and reading the result as X-in-12
Rise and run
Roof pitch compares vertical rise to horizontal run. In the US convention, run is standardized to 12 units, so a roof that rises 6 units for every 12 units of horizontal run is a '6-in-12' pitch. The units of rise and run cancel out, so a 6-in-12 pitch is 6-in-12 whether you measured in inches, feet, or centimetres — as long as rise and run use the same unit.
How to measure it
On an accessible section of roof (or in the attic along a rafter), mark a level, horizontal 12-inch (or 12-unit) run from a fixed point, then measure the vertical rise at the end of that run using a level and a tape measure or a rafter square. A digital angle finder or a smartphone level app gives a faster, though sometimes less precise, reading.
Pitch as an angle, and the slope factor
Pitch converts to an angle in degrees using arctan(rise/run) — a 6-in-12 pitch is arctan(6/12) ≈ 26.6°. It also produces the slope factor, √(1 + (rise/run)²), used to convert a flat footprint area into the true sloped roof area. Angle and slope factor are two different numbers derived from the same rise/run ratio — pitch itself never changes with the units you measured in.
Worked Example
A roof rises 8 inches over a 12-inch run.
Pitch = 8-in-12
Angle = arctan(8/12) ≈ 33.7°
Slope factor = √(1 + (8/12)²) ≈ 1.202
FAQ
- Is a steeper pitch always a bigger number?
- Yes — a higher rise-per-12 number means a steeper roof. A 3-in-12 is a shallow, walkable slope; a 12-in-12 (a 45° angle) is a steep roof requiring fall-protection for most work.
- Does roof pitch change if I measure in metric units?
- No. Pitch is a ratio of rise to run, so as long as both are measured in the same unit, the ratio — and the angle and slope factor derived from it — comes out identical whether you used inches, feet, or metres.