Understanding your result
Pitch, percentage slope, and angle describe the same geometry in different units. The sloped length uses your selected length unit and excludes overhangs and joinery allowances.
For other right-triangle dimensions, use the Pythagorean Theorem Calculator .
The formula
Rise is vertical and run is horizontal. The diagonal is the hypotenuse of that right triangle. A 12:12 pitch has equal rise and run, giving 45 degrees and 100% slope; percentage slope is not an angle in degrees.
Worked example
A rise of 3 m and a horizontal run of 6 m give a 6:12 pitch and 50% slope. The angle is about 26.565°, and the geometric sloped length is √45, approximately 6.708 m.
How to use this calculator
- Measure the vertical rise and horizontal run for the same roof section.
- Use one length unit for both measurements.
- Read the pitch, angle, and geometric length before applying project-specific allowances.
Run is not the full roof span
For a symmetric gable, each roof plane spans from an outside wall to the ridge. Its horizontal run is therefore half the wall-to-wall span before other details are considered. Entering the full span for one side halves the calculated slope. An asymmetric roof requires each plane to be measured and calculated independently.
Geometry is not a cutting list
A finished rafter can need extra length for an eave, ridge connection, and the chosen cuts. Structural loading, member size, material requirements, and minimum roofing slopes depend on the actual design and applicable specifications. The slope factor can convert a compatible horizontal projected area to a sloped plane area, but it does not add waste or account for intersecting hips and valleys.
Assumptions & limitations
What this calculation assumes
- A straight roof plane has uniform slope and the rise/run measurements describe one right triangle.
What to keep in mind
- No structural sizing, building-code approval, roofing-material selection, or joinery deduction.
Common questions
Is 50% slope the same as 50 degrees?
No. A 50% slope means a rise/run ratio of 0.5, which corresponds to about 26.565 degrees.
Does switching units convert the entered numbers?
No. The unit labels the measurements you enter. Enter equivalent values yourself when changing the measurement unit.