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Drain slope and fall calculator

Total fall = length of the run in feet × slope in inches per foot. A 40 ft run at 1/4 inch per foot falls 10 inches. Under IPC Table 704.1, horizontal drainage pipe 2-1/2 inches or smaller needs at least 1/4 inch per foot, 3 to 6 inches needs 1/8 inch per foot, and 8 inches or larger needs 1/16 inch per foot.

Code basis: International Plumbing Code (IPC) Table 704.1, minimum slope of horizontal drainage piping, as transcribed in DWV Pro. Not affiliated with or endorsed by the ICC. International Plumbing Code and IPC are trademarks of the International Code Council, Inc. Last checked 15 October 2026.

Run

Result

Enter the length of the run to see the total fall.

DWV Pro works out fall and inverts offline on iPhone with Vent & Slope.

Worked example

10.00 in of fall

Computed by the same code as the calculator above.

A 3" building drain runs 40 ft from the base of the stack to the foundation wall, at 1/4 inch per foot.

  1. Fall = run × slope: 40 ft × 1/4 in = 10.00 in, which is 10.00 in.
  2. Check the slope against IPC Table 704.1: pipe from 3 to 6 inches needs at least 1/8 inch per foot, so 1/4 inch per foot passes.
  3. At the bare minimum the same run would fall 5.00 in. If the end of the run has to come out lower or higher than that, the slope is the thing to change.
SlopeFall over 40 ft
1/16 inch per foot2.50 in
1/8 inch per foot5.00 in
1/4 inch per foot10.00 in
1/2 inch per foot1 ft 8.00 in

Limits, and when to ask your inspector

  • A planning aid, not a design or approval. The installer and the inspector are responsible for code compliance.
  • These are International Plumbing Code minimums. The Uniform Plumbing Code and local amendments can differ, so check which code your jurisdiction has adopted.
  • It works out fall for one straight run at one slope. It does not set invert elevations, size the pipe or check vents. DWV Pro does the sizing.
  • Some jurisdictions require more than the table minimum, for example 1/4 inch per foot on all building drains. Ask your inspector before you set grade.

Set inverts and size vents in DWV Pro

Invert elevations along the run, vent sizing, saved projects and a PDF sizing summary, offline on your iPhone. Free download with a one-time unlock.

Download DWV Pro on the App StoreDWV Pro details

Questions

What is the minimum slope for a 3-inch drain?

Under IPC Table 704.1, 1/8 inch per foot for pipe from 3 to 6 inches. Many plumbers still run 3-inch and 4-inch drains at 1/4 inch per foot where there is room, because a steeper building drain carries more fixture units.

How much fall is 1/4 inch per foot?

A quarter inch for every foot of run: 2-1/2 inches over 10 ft, 5 inches over 20 ft and 10 inches over 40 ft. Multiply the run in feet by 0.25.

Is there such a thing as too much slope?

The IPC sets minimums in Table 704.1. Some jurisdictions and manufacturers also limit how steep a horizontal drain may run, so check locally if a run is much steeper than 1/2 inch per foot.

Does slope change the pipe size?

For building drains and sewers, yes: IPC Table 710.1(1) lets a steeper drain carry more fixture units. Use the DWV pipe size calculator for that part.

What does the DWV Pro app add?

The app sizes drains free and offline. Vent & Slope, a one-time $4.99 purchase, adds slope and fall with invert elevations along the run, vent sizing from developed length, saved projects and a PDF sizing summary.

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