Roof Pitch Calculator

Convert roof pitch between rise over run, degrees and percent, and get the slope factors for figuring rafter lengths and roof area.

What Do You Know?
Level distance, not along the slope.
Roof Area
The flat area the roof covers, including overhangs. Works for any roof where every plane has the same pitch.
Fractions work too: try 7 1/2.
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Roof Pitch
Angle
Percent grade
Slope factor
Hip and valley factor
Type
Roof Area
Add a footprint to estimate

Roof List

Roofs you add are saved in this browser.

RoofPitchAngleFootprintRoof AreaSquaresRemove

How Roof Pitch Works

Roof pitch is how many inches a roof rises for every 12 inches it runs level, written as rise/12 (a 6/12 roof rises 6″ per foot). The same slope can be written as an angle or as a percent.

Pitch = rise ÷ run × 12   Angle = arctan(rise ÷ run)   Percent = rise ÷ run × 100
Slope factor = √(pitch² + 144) ÷ 12   Hip and valley factor = √(pitch² + 288) ÷ 12

Multiply a level distance by the slope factor to get the length along the roof. Multiply the roof's flat footprint by the slope factor to get the actual roof area. The hip and valley factor does the same for a hip or valley rafter on a roof where both sides share the same pitch.

Worked Example

Pitch Table

PitchAnglePercentSlope FactorHip / Valley

Tips on Pitch

Measure pitch with a level and tape: hold a 12″ level flat against the roof or rafter and measure straight down from the end to the roof. Asphalt shingles need at least 2/12 under the IRC, and roofs between 2/12 and 4/12 need two layers of underlayment. Steeper roofs above about 8/12 usually need roof jacks and harnesses to work on safely. A roofing square is 100 square feet of roof surface.