Pitch is a ratio — this page turns it into every number built from it
Roof pitch is rise over run, written as x/12. Everything a roofer or framer derives from it — the angle in degrees, the slope as a percentage, the slope factor that stretches a flat footprint into true roof area, and the length of a common rafter — is one right-triangle calculation. The Roofing Calculator on this site uses the same slope factor for materials; this page isolates the pitch maths itself, accepts any rise/run pair (not just per-12), and adds the rafter length for your span and overhang.
For a 6/12 pitch the factor is 1.118: every flat square foot becomes 1.118 ft² of roof, and every foot of horizontal run becomes 1.118 ft of rafter.
Worked example
A rise of 6 over a run of 12 is a 6/12 pitch: 26.57°, a 50% slope, with a slope factor of 1.1180. On a 24 ft wide building with a 12 in eave overhang, the horizontal run is 12 ft + 1 ft = 13 ft, so each common rafter is 13 × 1.1180 = 14.53 ft of line length before ridge and birdsmouth adjustments.
How to use this calculator
- Measure rise and run: put a level 12 inches out from the roof edge (gable ends and attic access are the safest spots) and read the vertical rise over that run — or enter any rise/run pair you measured another way.
- Read the pitch in x/12 form, the angle for your saw or speed square, and the slope percentage some codes and manufacturers quote.
- Use the slope factor with a footprint area to get true roof area — or jump to the Roofing Calculator, which does materials from there.
- Enter your building span and eave overhang for the common-rafter length; treat it as line length for estimating, not a finished cut list.
The standard pitches, for reference
4/12 = 18.43° and a 1.054 factor · 6/12 = 26.57° and 1.118 · 8/12 = 33.69° and 1.202 · 10/12 = 39.81° and 1.302 · 12/12 = 45° and 1.414. Below about 3/12, standard shingles give way to low-slope systems; above 12/12 you are in specialist-access territory.