Drain slope calculator
Drainage runs on gravity and on nothing else. Too little fall and the solids stop while the water carries on; too much and the water leaves the solids behind. The code sets a minimum by pipe size and leaves the rest to you. Enter the size, the slope and the run, and this returns the total fall you have to build into the hangers.
Pre-filled with a 24 ft 3" branch at the code minimum. Change any field.
Total fall over the run
- Code minimum for this size
- 1/8"per ft
- Slope you entered
- 1/8"per ft
- Fall per 10 ft
- 1.25in
- Invert drop
- 0.25ft
| Size | Minimum slope | Fall over your run |
|---|---|---|
| 1-1/4" | 1/4" / ft | 6.0" |
| 1-1/2" | 1/4" / ft | 6.0" |
| 2" | 1/4" / ft | 6.0" |
| 2-1/2" | 1/4" / ft | 6.0" |
| 3" | 1/8" / ft | 3.0" |
| 4" | 1/8" / ft | 3.0" |
| 6" | 1/8" / ft | 3.0" |
| 8" | 1/16" / ft | 1.5" |
| 10" | 1/16" / ft | 1.5" |
How it sizes
Slope is stated as a rise over a foot, so the arithmetic is trivial: fall equals slope times run. The judgement is in the number you choose. IPC Table 704.1 sets 1/4" per foot for anything 2-1/2" and under, 1/8" for 3" through 6", and 1/16" at 8" and above. Larger pipe gets less slope because it carries a deeper stream at the same flow, and depth is what moves solids.
The one exception the code writes down is grease. Anything upstream of a grease interceptor runs at not less than 1/4" per foot regardless of size, because congealing grease needs velocity that a shallow grade cannot give it.
Worked example
A 3" branch runs 24 ft from a bathroom group to the stack. At the 1/8" minimum:
| Quantity | Value |
|---|---|
| Run | 24 ft |
| Slope | 1/8" / ft |
| Total fall | 3.0 in |
| Invert drop | 0.25 ft |
Three inches over 24 ft. That is the number that has to fit between the joists and the ceiling below, and it is the number that decides whether the run clears a beam. Work it out before the holes are drilled, not after.
Uniform, not average
Code notes
IPC 704.1 sets the minimum by size. UPC covers the same ground with slightly different language and its own table; on a house branch the numbers land in the same place. Which one applies is set by your authority having jurisdiction (AHJ) — see the IPC vs UPC comparison.