About the Roof Pitch Calculator
Roof pitch describes the steepness of a roof and is expressed as a ratio of vertical rise to horizontal run. A 6/12 pitch means the roof rises 6 inches for every 12 inches of horizontal distance. Knowing your roof pitch is essential for ordering roofing materials, calculating rafter lengths, and ensuring proper water drainage and snow shedding.
To measure roof pitch in the field, place a level horizontally against the roof surface, measure 12 inches along the level, then measure the vertical distance from the end of the level down to the roof surface. That vertical measurement is your rise per 12 inches of run. Alternatively, you can measure from inside the attic using the rafters.
The pitch multiplier (also called the roof slope factor) is critical for calculating actual roof area from the footprint area. A flat roof has a multiplier of 1.0, while a 12/12 pitch has a multiplier of 1.414, meaning the actual roof surface area is 41.4% larger than the footprint. This directly affects how many shingles, sheets of underlayment, and other roofing materials you need to order.