Roof Pitch Calculator

How roof pitch is calculated

Roof pitch is the ratio of vertical rise to horizontal run, conventionally stated per 12 inches of run:

Pitch (per 12) = rise ÷ run × 12
Angle = arctan(rise ÷ run)
Rafter length per foot of run = √(12² + pitch²) inches

So a roof that rises 6 inches over a 12-inch run is a "6/12 pitch" — its angle is about 26.6°, and every foot of horizontal run needs about 13.42 inches of rafter. The slope factor converts flat (plan-view) area to true sloped roof area: multiply the building footprint's roof-plan area by the slope factor to order shingles and underlayment. A 6/12 roof's factor is about 1.118, meaning roughly 12% more material than the flat measurement suggests.

Measuring an existing roof: in the attic or at a gable end, hold a level out 12 inches horizontally from the rafter or wall, then measure straight down (or up) from the end of the level to the roof surface — that's the rise. From the ground, a pitch-gauge app or an angle finder on the rake gives the angle, which this calculator converts back to x/12 terms.

Common pitches: 3/12 (about 14°) is near the minimum for standard asphalt shingles; 4/12–6/12 covers most tract homes; 8/12–12/12 (up to 45°) is steep, sheds snow well, and is harder and costlier to reroof. Metal and specialty roofing have their own minimum-pitch rules — check the manufacturer's spec for your material.