How to use
- Choose the measurement you know: rise and horizontal run, rise per 12, angle in degrees or percent grade.
- Enter the value in the indicated units. If you need actual slope length, enable the size inputs and supply the horizontal run.
- Optionally enter projected area to see its equal-pitch surface-area equivalent.
- Read the rise/run diagram and check the distinction between ratio, degrees and percent before transferring values to another tool.
Calculation method
Let t = rise / horizontal run. Pitch per 12 = 12t, percent grade = 100t and angle = arctan(t). Slope multiplier = √(1 + t²). When horizontal run is known, slope length = run × multiplier. A projected area on one uniform pitch uses the same multiplier to obtain the slope surface equivalent. Metric/US changes physical length display without altering the ratio or angle.
Worked example
A rise of 1.5 m over a 3 m horizontal run gives t = 0.5: 6:12 pitch, 50% grade and approximately 26.565°. The slope length is √(3² + 1.5²) ≈ 3.3541 m. A 30 m² projected area at that pitch corresponds to about 33.5410 m² of surface area. A 50% grade is not a 50-degree angle.
Assumptions and limits
The diagram explains geometry and is not a construction drawing. The tool does not specify a suitable roof pitch, drainage design, rafter section, loading, ridge detail or covering system. Run is horizontal, not measured along the rafter. The optional area calculation assumes one consistent pitch; complex roof areas require separate treatment.
Continue your plan
Use the roofing material calculator for pack quantities, the area calculator for measured sections, and the lumber calculator for separately specified timber.
Copy, download or print a valid result to keep the inputs with the estimate. Currency selections label your prices; they do not apply an exchange rate. Product-dependent examples should be replaced with data for the material or equipment you actually use.
For the measurement and planning details, read understanding roof pitch, degrees, grade and run.