IPC 704.1 · UPC 708.0
Pipe slope calculator.
Work out drainage fall in inches per foot, total drop over a run, and percent grade — then check it against the IPC or UPC minimum for the pipe size you are running.
Fractions work: type 1/4 or 1/8 rather than a decimal.
Units welcome: 40 ft, 12 m.
Common Falls
Meets IPC minimum
Meets the IPC minimum of 1/8″ per foot for 4″ pipe.
This is steeper than about ½″ per foot. Liquid can outrun the solids at that fall, so a horizontal branch is usually kept gentler.
Total Drop
10″
Fall per Foot
1/4″ per ft
Percent Grade
2.08%
20.8‰
Code Minimum
1/8″ per ft
Minimum Drop Over Run
5″
Minimum Slope by Pipe Size
Where the two codes differ, the row is marked. Local amendments override both.
| Size | IPC | UPC | IPC Grade |
|---|---|---|---|
| 1.5″ | 1/4″ | 1/4″ | 2.08% |
| 2″ | 1/4″ | 1/4″ | 2.08% |
| 2.5″ | 1/4″ | 1/4″ | 2.08% |
| 3″ | 1/8″ | 1/4″ | 1.04% |
| 4″ | 1/8″ | 1/8″ | 1.04% |
| 5″ | 1/8″ | 1/8″ | 1.04% |
| 6″ | 1/8″ | 1/8″ | 1.04% |
| 8″ | 1/16″ | 1/8″ | 0.52% |
| 10″ | 1/16″ | 1/8″ | 0.52% |
| 12″ | 1/16″ | 1/8″ | 0.52% |
Why Drains Need a Specific Slope
A drain has two jobs at once: move the liquid, and carry the solids along with it. Too little fall and nothing moves at all. Too much fall and the liquid accelerates away, leaving the solids stranded on the invert to build up over time. The code minimums exist to keep flow in the band where both travel together.
That is why the required slope gets gentler as pipes get larger, which surprises people who expect the opposite. A bigger pipe carries more volume at the same depth of flow, so it needs less gradient to reach a self-cleansing velocity.
The Numbers
IPC Table 704.1 sets the minimum fall for horizontal drainage piping at ¼ inch per foot for pipes 2½ inches and smaller, ⅛ inch per foot for 3 to 6 inch, and 1/16 inch per foot for 8 inch and larger. Piping upstream of a grease interceptor is held to ¼ inch per foot regardless of size.
UPC 708.0 draws the line in a different place: ¼ inch per foot for pipe 3 inches and smaller, with ⅛ inch per foot permitted for larger sizes subject to approval. Three-inch pipe is the one size where following the wrong code gives you the wrong answer — and it happens to be a very common residential size.
Neither figure is the final word. Jurisdictions amend model codes freely, and your local authority has the last say. Use the number here to plan and to sanity-check, then confirm against the code actually adopted where you are building.
Planning the Total Drop
The number that decides whether a design works is rarely the slope itself — it is the total drop over the run. At ¼ inch per foot, a 40-foot run falls 10 inches. Over 80 feet it falls 20 inches. That vertical budget has to fit between where the pipe leaves the fixture and where it has to arrive, and on a long run under a slab it is frequently the constraint that forces a redesign.
Work it out at the start. Finding out at rough-in that the last twenty feet of a run has nowhere left to fall is an expensive discovery.
Common Questions
What slope should a drain pipe have?
Under the International Plumbing Code, pipes 2½ inches and smaller need at least ¼ inch of fall per foot, 3 to 6 inch pipes need ⅛ inch per foot, and 8 inch and larger need 1/16 inch per foot. The Uniform Plumbing Code differs: it requires ¼ inch per foot up to 3 inch pipe.
Is ¼ inch per foot the same as 2%?
Almost exactly. A quarter inch of fall over twelve inches of run is 0.25 ÷ 12 = 2.083%. It is usually spoken of as a 2% slope, and the difference is far smaller than the tolerance you can hold while laying pipe.
Can a drain pipe be too steep?
It can, though this is a rule of thumb rather than a code limit in most jurisdictions. If the pipe is very steep the liquid can outrun the solids, leaving them behind to build up. Fall much beyond about ½ inch per foot in a horizontal branch is generally avoided for that reason.
Why do IPC and UPC disagree about 3 inch pipe?
They are separate model codes written by different bodies, and 3 inch is exactly where their tables diverge — the IPC groups it with 4 and 6 inch at ⅛ inch per foot, while the UPC groups it with the smaller sizes at ¼ inch per foot. Which one applies depends on what your jurisdiction has adopted.
How much total drop do I need over a run?
Multiply the fall per foot by the length of the run. At ¼ inch per foot, a 40 foot run drops 10 inches. That total is what has to be accommodated between the fixture connection and the point of discharge, so it is worth checking early in a design.
Does the slope apply to vertical stacks?
No. These minimums apply to horizontal drainage piping. Vertical stacks fall under gravity and do not have a slope requirement, though the fittings that turn a stack back to horizontal do have their own rules.