Pipe drop = length × slope. A 50 ft drain laid at the minimum 1/4 inch per foot falls 12.5 inches, a 2.08% slope or 1.19 degrees. That 1/4 in per ft is the minimum for drain pipe up to 2 1/2 inches in the International Plumbing Code (IPC) and for most pipe under the Uniform Plumbing Code (UPC).
How the pipe slope is calculated
Slope is fall over run. Convert the inches of fall per foot to a ratio, then multiply by the length.
slope = inches of fall per foot ÷ 12
total drop = length × slope; percent = slope × 100; degrees = arctan(slope)
Worked example: a 50 ft drain
| Slope | Total drop over 50 ft | Percent | Degrees | Ratio |
|---|---|---|---|---|
| 1/4 in per ft | 12.5 in | 2.08% | 1.19° | 1 : 48 |
| 1/8 in per ft | 6.25 in | 1.04% | 0.60° | 1 : 96 |
| 1/16 in per ft | 3.13 in | 0.52% | 0.30° | 1 : 192 |
Drain pipe slope by size (IPC)
Section 704.1 of the 2021 International Plumbing Code, and Table 704.1 in it, set the minimum slope of horizontal drainage pipe by diameter:
| Pipe diameter | Minimum slope |
|---|---|
| 2 1/2 in or less (1 1/2 in and 2 in branches) | 1/4 in per ft (2%) |
| 3 in to 6 in (toilet branch, 4 in main) | 1/8 in per ft (1%) |
| 8 in or larger | 1/16 in per ft |
The same section says pipe upstream of a grease interceptor must not go below 1/4 in per ft. Choose your pipe diameter in the calculator and the code check compares your slope with the table.
Plumbing drain slope under the UPC
The Uniform Plumbing Code, where it is adopted, asks for 1/4 in per ft (2%) on horizontal drains. Section 708 permits 1/8 in per ft (1%) for pipe 4 in and larger when it is impractical to reach 1/4 in and the authority having jurisdiction approves it. The calculator's UPC mode sets the minimum to 1/4 in per ft and shows that allowance for pipe 4 in and up. Codes are adopted with local changes, so ask your building department which edition applies.
Sewer pipe slope
The sewer line from the house to the street or septic tank is horizontal drainage pipe too, and the same table is the usual starting point; the "Sewer line" mode is for long runs where a small change in slope moves the total drop by many inches. A 100 ft run at 1/4 in per ft drops 25 in, over 2 ft, so plan the depth at the far end before you dig. Your local utility or health department can set its own minimum for the sewer.
Drop by distance at 1/4 inch per foot
| Run | 1 ft | 2 ft | 5 ft | 10 ft | 25 ft | 50 ft | 100 ft |
|---|---|---|---|---|---|---|---|
| Drop (in) | 0.25 | 0.5 | 1.25 | 2.5 | 6.25 | 12.5 | 25 |
For 1/8 in per ft halve each number; for 1/16 in per ft quarter it. The code sets minimums; this calculator does not set a maximum, and some local codes do.
The next questions
- Sizing the tank at the end of the line? Use the septic tank size calculator.
- Other slopes on the same job: the gutter slope calculator and the driveway slope calculator.
- Buying pipe? Add up the runs with the linear feet calculator.
- Supply side of the job: a PEX calculator for the tubing and a water pressure calculator for the fixtures.
- See every heating, cooling and plumbing calculator.