Calculators

Roof pitch calculator

Safety verdict: Safe from the ground measured inside the attic against a rafter, or from a level and tape at the gable end

Rise (in)Vertical

Run (in)Horizontal, usually 12

Convert rise and run into the X:12 pitch a roofer will quote, the angle in degrees, and the multiplier that converts footprint into roof area.

The formula

pitch = (rise / run) x 12 ; angle = arctan(rise / run) ; multiplier = sqrt(rise^2 + 12^2) / 12

Pitch is written as rise over a 12-inch run. A 6:12 roof rises 6 inches for every 12 inches of horizontal travel.
You can measure this from inside the attic with a level and a tape against a rafter. Nobody needs to be on the roof.
Below about 2:12, asphalt shingles are not watertight and a low-slope membrane is the correct covering. That is a code and physics limit, not a preference.
At 8:12 and above the ice barrier requirement changes, and the roof stops being walkable for most people.