Plumbing / Free field reference
IPC Pipe Slope Table & Drain Fall Calculator
2021 IPC Table 704.1 sets minimum slopes of ¼ inch per foot for pipe 2½ inches or smaller, ⅛ inch per foot for 3–6 inches, and ¹⁄₁₆ inch per foot for 8 inches or larger. Upstream of a grease interceptor, use at least ¼ inch per foot.
Scope: 2021 International Plumbing Code, Section and Table 704.1. Horizontal drainage piping only. Confirm local adoption and amendments; this is not a UPC, storm drainage, or vent-sizing tool.
Quick reference
| Nominal pipe size | Minimum slope | Approx. slope |
|---|---|---|
| 2½ in or smaller | ¼ in per ft | 2.08% |
| 3–6 in | ⅛ in per ft | 1.04% |
| 8 in or larger | ¹⁄₁₆ in per ft | 0.52% |
| Upstream of grease interceptor | ¼ in per ft (all sizes) | 2.08% |
Before you use the result
- Total fall in inches = horizontal run in feet × slope in inches per foot. A 20 ft run at ¼ in per ft needs 5 in of fall.
- Measure horizontal run, not diagonal pipe length. Maintain a uniform slope; total fall alone does not establish that the installed pipe has no flat spots.
- This checks slope arithmetic only. Pipe capacity, fixture units, venting, cleanouts, and available elevations require separate checks.
Sources & edition notes
This is an independent HandySuite reference. The linked publisher’s code or equipment documentation governs the work.
Common questions
What is the minimum slope for a 3-inch drain under this IPC table?
⅛ inch per foot, except upstream of a grease interceptor where at least ¼ inch per foot is required. Confirm the locally adopted code.
How much fall does 20 feet of 2-inch drain need?
At ¼ inch per foot, 20 feet needs 5 inches of fall. This is the slope calculation only.