Find roof pitch, angle, percent slope, and the slope multiplier from rise and run — or work backward from a known angle.
Set the input method, then enter either the rise and run or a known angle in degrees. You get the pitch as X-in-12, the angle in degrees, the slope as a percent grade, and the slope multiplier that converts a flat footprint into true sloped roof area. It works the same for a main roof, a shed or lean-to roof, a porch, or a pergola — pitch is just rise over run regardless of the structure.
How it works
Pitch is the rise in inches per 12 inches of horizontal run, written X:12. The angle is the arctangent of rise ÷ run, and percent grade is rise ÷ run × 100. The slope multiplier is √(rise² + run²) ÷ run — multiply your roof's flat footprint area by it to get the true sloped surface area for a shingle estimate. If you only know the angle, switch the input method to degrees and the calculator runs the conversion in reverse. Common pitches for reference: 4:12 is 18.4°, 6:12 is 26.6°, 8:12 is 33.7°, and 12:12 is a 45° roof.
Frequently asked questions
How do I calculate roof pitch?
Pitch is the rise in inches for every 12 inches of horizontal run. A roof that rises 6 inches over 12 inches of run is a 6:12 pitch. Measure 12 inches out along a level from the roof surface, then measure straight down to the roof at that point — that drop is your rise.
How do I convert a roof angle to pitch?
Switch the input method to degrees and enter the angle. The pitch is 12 × tan(angle): a 30° roof is about 6.9:12, and a 45° roof is exactly 12:12. To go the other way, the angle is the arctangent of rise ÷ run.
What is the angle of a 6:12 roof?
About 26.6 degrees, or a 50% grade. The angle is the arctangent of rise divided by run.
What is the slope multiplier for?
Multiply your roof's flat footprint area by the slope multiplier to get the actual sloped surface area — the number you feed into a shingle estimate. A 6:12 roof has a multiplier of about 1.118, so 1,000 sq ft of footprint is roughly 1,118 sq ft of roof.
Does this work for a shed or lean-to roof?
Yes. A shed, lean-to, pergola, or flat-ish roof is still just rise over run — enter the rise and run of the single slope, or the angle, and you get the same pitch, grade, and multiplier.