Roof Slope Calculator: Find Rise, Run, Pitch and Angle
Divide vertical rise by horizontal run to find roof slope. Multiply by 12 for the common x:12 pitch format.
What to measure
Measure rise vertically from a level baseline to the roof surface. Measure run horizontally, not along the rafter. For one side of a symmetrical gable roof, the run is commonly measured from the outside wall line to the ridge centerline.
Use matching units before dividing. Inches and inches work; feet and feet work. Do not mix feet with inches.
Convert roof slope to the format you need
Slope ratio = rise / runThe basic roof slope.Pitch = (rise / run) × 12The x:12 pitch format.Slope percent = (rise / run) × 100Percent grade.Angle = atan(rise / run)Convert radians to degrees when needed.Rafter = √(rise² + run²)The diagonal for that rise and run.Worked example: a 6:12 roof pitch
With a rise of 6 inches over a run of 12 inches, the slope ratio is 6 / 12 = 0.5. Multiply by 12 to get a 6:12 pitch. The same ratio is 50% slope. The angle is approximately 26.57 degrees, and the diagonal for that 6-by-12 triangle is about 13.42 inches.
That diagonal is the geometry for the stated rise and run only. It does not automatically include overhangs, ridge-thickness adjustments, birdsmouth cuts, or material allowances.
A quick field-check method
- Establish a level reference line against the roof.
- Mark a 12-inch horizontal run.
- Measure vertically from that mark to the roof surface.
- The vertical measurement is the first number of the x:12 pitch.
For a longer baseline, scale it back to a 12-inch run. For example, 10 inches of rise over 24 inches of run is (10 / 24) × 12 = 5, or a 5:12 pitch.
Roof slope calculator FAQ
Is roof slope the same as roof pitch?
They describe the same geometry in different formats. Slope is rise divided by run. Pitch is often written as the rise per 12 inches of horizontal run, such as 4:12 or 6:12.
Why is run not the same as rafter length?
Run is horizontal distance. Rafter length is the diagonal distance along the roof plane, so they are different sides of the roof triangle.
Can I calculate roof angle from slope?
Yes. Divide rise by run, then take the inverse tangent. Convert the result to degrees if your calculator does not already display degrees.
Before using a measurement for construction
This calculator-style method is useful for geometry and planning. Verify structural details and local code requirements with an appropriate qualified professional before ordering materials or cutting framing members.