How drain slope is calculated
Drain slope is given in inches of fall per foot of horizontal run. Multiply the run by the slope and you have the total drop between the two ends of the pipe. The minimum slope comes from the pipe size.
minimum slope = IPC Table 704.1 value for the pipe size
slope used = your choice, or the minimum when you pick "Code minimum"
total drop (in) = run (ft) × slope (in per ft)
total drop (ft) = total drop (in) ÷ 12
The same slope can be written as a percent grade or an angle, which is what a laser level or a digital level reads:
grade (%) = slope ÷ 12 × 100
angle (°) = arctan(slope ÷ 12)
The run is the level distance between the two ends, not the length of pipe you cut. At drain slopes the two differ by a small fraction of an inch, so either measurement gives the same drop to the nearest 1/16 in.
Worked example: 20 ft of 2 in pipe at 1/4 in per foot
The calculator opens on 20 ft of 2 in drain at 1/4 in per foot.
- Minimum slope: 2 in pipe is in the "2-1/2 inches or less" row of IPC Table 704.1, so the minimum is 1/4 in per foot. The chosen 1/4 in per foot meets it.
- Total drop: 20 ft × 1/4 in per ft = 5", or 0.417 ft.
- Grade: 1/4 ÷ 12 × 100 = 2.08%.
- Angle: arctan(1/4 ÷ 12) = 1.19°.
If the high end of the pipe sits at the bottom of a floor joist, the low end must be 5" lower at the stack or the cleanout.
Minimum slope by pipe size
IPC 2021 Table 704.1 sets three minimums for horizontal drainage piping:
- 2-1/2 inches or less: 1/4 in per foot (2.08%)
- 3 to 6 inches: 1/8 in per foot (1.04%)
- 8 inches or larger: 1/16 in per foot (0.52%)
The Uniform Plumbing Code is stricter. UPC 2021 Section 708.1 calls for 1/4 in per foot on all sizes, with 1/8 in per foot allowed on 4 in and larger pipe only with prior approval. Larger pipe is allowed a flatter slope because flow in a bigger pipe reaches a self-cleaning speed at a lower grade. Your jurisdiction adopts one code, often with amendments, so check which one is in force. The table below converts the common slopes and shows the block to put under a 2 ft level.
| Slope (in/ft) | Grade | Angle | Fall per 10 ft | Fall per 50 ft | Block, 2 ft level |
|---|---|---|---|---|---|
| 1/16 | 0.52% | 0.30° | 5/8" | 3-1/8" | 1/8" |
| 1/8 | 1.04% | 0.60° | 1-1/4" | 6-1/4" | 1/4" |
| 1/4 | 2.08% | 1.19° | 2-1/2" | 1' 0-1/2" | 1/2" |
| 1/2 | 4.17% | 2.39° | 5" | 2' 1" | 1" |
Measuring and setting the slope
- Measure on the invert, the bottom inside of the pipe. On the outside, the top or bottom of the pipe works as long as the pipe and fittings are the same size along the run.
- For a quick check, tape a block to one end of a 2 ft level: 1/2 in for 1/4 in per foot, 1/4 in for 1/8 in per foot. Put the block on the low side and center the bubble.
- On long runs, set the two ends first and pull a string line between them, then hang the pipe to the string. A 1/4 in slope over 40 ft is 10"; at 1/8 in it is 5".
- Work backward from the outlet. Find the invert of the stack, building drain or sewer connection, add the total drop, and confirm the high end still fits under the floor or above the footing.
- Keep the slope uniform. A belly or a flat spot collects solids even when the average slope is correct, so check at every hanger.
- Too much slope is allowed, but over 1/2 in per foot ask your inspector, and use 45° fittings to take a larger drop rather than tilting the whole run.
Building sewers outside the foundation and vent piping follow their own sections of the code. Hire a licensed plumber when the run ties into a sewer or septic system, or when the required fall does not fit the space you have.
Sources
- International Code Council, 2021 International Plumbing Code, Table 704.1 (slope of horizontal drainage piping)
- IAPMO, 2021 Uniform Plumbing Code, Section 708.1 (grade of horizontal drainage piping)
- Geometry: drop = run × slope; percent grade = slope ÷ 12 × 100; angle = arctan(slope ÷ 12)