Roof pitch formula
Roof pitch is rise divided by horizontal run. In U.S. construction it is commonly normalized to a 12-inch run. The same ratio gives the roof angle, slope percentage, and pitch factor.
Worked example: 6:12 roof
A 6 in rise over a 12 in run is a 6:12 pitch. The angle is approximately 26.57°, the grade is 50%, and the slope factor is approximately 1.118. If the building span is 24 ft with a 1 ft horizontal overhang, the common-rafter line from wall centerline geometry is based on 13 ft of horizontal run multiplied by the slope factor.
Pitch, angle, slope and factor are different numbers
- Pitch: rise normalized to 12 units of horizontal run.
- Angle: the roof line measured in degrees from horizontal.
- Slope percentage: rise divided by run, multiplied by 100.
- Pitch factor: sloped length divided by horizontal run; useful for area and length conversions.
Rafter length is geometry—not a cut list
The optional rafter result is the theoretical sloped line from half-span plus the horizontal overhang. Real rafter layout also depends on ridge thickness, birdsmouth geometry, fascia/subfascia, tail details, and how the building is framed. Use the result as geometry, not as a finished cut length.
Roof pitch calculator FAQ
What angle is a 6:12 roof?
Approximately 26.57 degrees.
What is the multiplier for a 6:12 roof?
Approximately 1.118. Multiply horizontal run or footprint area by the factor when the geometry is a simple uniform slope.
Can I enter a run other than 12 inches?
Yes. The calculator accepts any rise and run and normalizes the result to an equivalent x:12 pitch.