Roof Pitch Calculator
Free roof pitch calculator — angle, x-in-12 pitch, slope factor, and grade from roof rise and run.
Everyday Uses
Roofing projects
Get the angle and slope factor to size rafters and materials.
Rise & run
Convert between rise-in-12 pitch and degrees instantly.
Material length
Use the slope factor to turn plan length into true roof length.
Drainage
Check a slope sheds rain and snow the way it should.
Frequently Asked Questions
How do you calculate roof pitch?
Roof pitch is the rise (vertical) over the run (horizontal), usually written as "rise-in-12". Divide the rise by the run and multiply by 12: a roof that rises 5 units over a 12-unit run is a 5-in-12 pitch. The angle is arctan(rise ÷ run) — a 5-in-12 pitch is about 22.6°.
What is the slope factor and why does it matter?
The slope factor is √(rise² + run²) ÷ run — the ratio between the true sloped length of the roof and its flat (plan) length. Multiply a horizontal measurement by it to get the actual roof-surface length. A 5-in-12 roof has a slope factor of about 1.083, so its surface is 8.3% longer than the footprint, which affects how much roofing material you buy.
What is a good roof pitch?
Most residential roofs fall between 4-in-12 and 9-in-12 (about 18° to 37°). Low-slope roofs below 3-in-12 need special membranes to stay watertight, while steep roofs shed water and snow well but are harder and costlier to work on.