How much slope does a drain pipe need?
Pick the pipe and the length, get the fall the code asks for, or check the slope you have.
Nominal drain pipe size in inches, as the codes list it.
Measured horizontally, along the pipe, not up the slope.
What is the minimum slope for a drain pipe?
The 2021 and 2024 International Plumbing Code (Table 704.1) asks for at least ¼ inch per foot on drain pipe 2½ inches and smaller, ⅛ inch per foot on pipe from 3 to 6 inches, and 1/16 inch per foot on pipe 8 inches and larger. In other terms ¼ inch per foot is 2.08% (1 : 48), ⅛ inch per foot is 1.04% (1 : 96) and 1/16 inch per foot is 0.52% (1 : 192); the code calls the first a “2-percent slope”. Piping upstream of a grease interceptor needs at least ¼ inch per foot at any size.
How much fall do I need for a drain run?
Multiply the run in feet by the slope in inches per foot. A 4-inch drain run 40 feet at ⅛ inch per foot needs 40 × ⅛ = 5 inches of fall; a 2-inch branch 10 feet long at ¼ inch per foot needs 2½ inches. The calculator also works backward: with 5 inches of drop available, a 4-inch pipe at ⅛ inch per foot can run 40 feet.
Is the UPC slope rule different from the IPC?
Yes. The Uniform Plumbing Code, section 708.1, asks for ¼ inch per foot (2%) on horizontal drainage piping of every size. Pipe 4 inches and larger may be laid at ⅛ inch per foot (1%) only where ¼ is impractical because of the depth of the street sewer or the structure, and only when the authority having jurisdiction approves it first. So a 3-inch drain needs ⅛ inch per foot under the IPC but ¼ inch per foot under the UPC. Which code applies depends on your state or city.
Can a drain pipe have too much slope?
Section 704.1 and Table 704.1 of the IPC give minimum slopes only; neither sets a maximum. The IPC also defines any pipe that makes an angle of 45 degrees or more with the horizontal as vertical pipe, which is a different case from the table. Some local codes add their own limits, so ask your building department before running a very steep horizontal drain.
How do I check the slope of a pipe I already installed?
Measure the drop between two points along the pipe and the horizontal distance between them, and divide: slope = drop ÷ length. A 4-inch pipe that falls 4 inches over 40 feet has a slope of 0.1 inch per foot (0.83%), which is flatter than the ⅛ inch (0.125) the IPC asks for. At the minimum it would fall 5 inches, so 1 inch more. Pick “Check my slope” to do the arithmetic and see the verdict.
Sources: International Plumbing Code 2021 and 2024, section 704.1 and Table 704.1, and the Chapter 2 definitions of horizontal and vertical pipe (ICC, codes.iccsafe.org, free view); Uniform Plumbing Code section 708.1, read in the 2016 California Plumbing Code and the 2021 Hawaii Plumbing Code, which adopt it (IAPMO’s own text is for members). Slope in percent, degrees and ratio is arithmetic. Slope of a yard, drive or ramp? Use the slope calculator; draining a trench? See the French drain calculator; water held in the pipe: pipe volume calculator and friction loss calculator. How we check our calculators.