Quick answer
Multiply the horizontal run by the slope as a decimal to find the vertical drop. At a 0.5% slope, a 4 m run drops 20 mm. Use the same unit for the run and the drop before converting the result.
Formula and assumptions
Pitch (%) = 100 × drop ÷ run. Run and drop must use the same unit. Verify the manufacturer’s pitch requirements.
Worked example
A 10 m run at 0.25% drops 0.025 m (25 mm).