Calculate drainage fall
Formula and examples
Fall in inches = run in feet × 12 × slope percent ÷ 100
The ratio is approximately 1 to the result of 100 divided by the slope percentage.
A 2 percent slope across 20 feet requires 4.8 inches, or about 122 millimetres, of total fall. That is equivalent to a 1:50 ratio. A 1 percent slope over the same run would produce 2.4 inches of fall.
Use the result as part of a drainage plan
- Measure the horizontal run rather than the sloped surface distance.
- Establish a known high point and a safe, permitted discharge location.
- Check door thresholds, foundations, retaining edges and neighbouring grades.
- Account for finished surface thickness and compaction when setting base elevations.
Last updated August 28, 2026. Site drainage must comply with local requirements and should be reviewed when runoff, foundations or neighbouring properties are involved.