Calculator
Plumbing Drain Pipe Slope Calculator
Work out drain slope three ways: fall from a known run, slope from a measured fall, or the longest run your available fall will cover. Every answer comes back in inches of fall, in/ft, percent grade and 1:x ratio, with start and end invert elevations, a PASS/FAIL against IPC Table 704.1, and a half-full velocity check. Free, no signup.
Free
SlopeEnter it however the drawing shows it, inches of drop per foot, percent grade, or a 1:x ratio. Switching units converts the value.
Pipe material and invert elevation
4" drainIPC min 1/8"/ft3.72 ft/s half full
Total fall over the run
10"
0.25 in/ft · 2.08% · 1:48 over 40 ft
Meets code minimum and self-cleansing velocity
Slope at a glance
Slope
0.25 in/ft2.08% · 1:48.0
Total fall
10"0.833 ft over a 40 ft run
IPC minimum slope
1/8"/ft3" to 6" · 1.04% · PASS
Max run at that minimum
80 ftWhat 10" of fall buys you at 1/8"/ft
Velocity, half full
3.72 ft/sManning n 0.011 · clears the 2 ft/s scouring floor
Flow, half full
72.9 GPM145.7 GPM full bore
Start invert
0.000 ftUpstream end of the run
End invert
-0.833 ftDown 0.833 ft from the start
Industry benchmarks
| Slope | Grade | Ratio | Where it applies |
|---|---|---|---|
| 1/16 in/ft | 0.52% | 1:192 | IPC minimum for 8" and larger. |
| 1/8 in/ft | 1.04% | 1:96 | IPC minimum for 3" to 6". |
| 1/4 in/ft | 2.08% | 1:48 | IPC minimum for 2-1/2" and smaller; UPC default for everything. |
| 1/2 in/ft | 4.17% | 1:24 | Practical ceiling, steeper and liquid outruns solids. |
IPC Table 704.1, horizontal drainage piping 3" to 6" requires a minimum slope of 1/8 in/ft.
UPC 708.0 is stricter: 1/4 in/ft on all horizontal drainage piping, with 4" and larger permitted down to 1/8 in/ft only where the AHJ approves it.
The 2 ft/s scouring floor is a design screening check drawn from UPC-lineage codes and ASPE practice, not from IPC 704.1, the 2021 IPC exception under Table 704.1 is the grease-interceptor rule (1/4 in/ft upstream of an interceptor).
Above 1/2 in/ft (4.17%) liquid can outrun solids and strand them in the line. This is trade practice, not a code maximum.
Minimum slopes cited from the 2021 International Plumbing Code. Velocity is Manning's equation at half-full flow, where the hydraulic radius is D/4. Always confirm against the code edition your AHJ has adopted.
What you'll get
Pick the pipe size and what you are solving for
Nominal size sets the IPC Table 704.1 minimum, 1/4 in/ft up to 2-1/2 inches, 1/8 in/ft for 3 to 6 inches, 1/16 in/ft at 8 inches and larger. Then choose fall from run, slope from fall, or max run.
Enter the run, slope, or measured fall
Slope takes in/ft, percent, or a 1:x ratio and converts between them as you switch units. Fall takes inches or feet. Add a start invert elevation to get the downstream invert directly.
Check it against code and velocity
See the slope in every unit, the total fall, the longest run that fall allows at the code minimum, and the half-full Manning velocity flagged against the 2 ft/s scouring floor and the 1/2 in/ft over-steep ceiling.
How it works
- 1
Pick the pipe size and what you are solving for
Nominal size sets the IPC Table 704.1 minimum, 1/4 in/ft up to 2-1/2 inches, 1/8 in/ft for 3 to 6 inches, 1/16 in/ft at 8 inches and larger. Then choose fall from run, slope from fall, or max run.
- 2
Enter the run, slope, or measured fall
Slope takes in/ft, percent, or a 1:x ratio and converts between them as you switch units. Fall takes inches or feet. Add a start invert elevation to get the downstream invert directly.
- 3
Check it against code and velocity
See the slope in every unit, the total fall, the longest run that fall allows at the code minimum, and the half-full Manning velocity flagged against the 2 ft/s scouring floor and the 1/2 in/ft over-steep ceiling.
Why drain slope is a two-sided problem
Every plumber knows the quarter-inch-per-foot rule, but IPC Table 704.1 only requires that on pipe 2-1/2 inches and smaller. A 3-inch to 6-inch drain needs 1/8 inch per foot, and 8 inches and larger needs only 1/16 inch per foot. Larger pipe gets a flatter minimum because it carries a deeper stream at the same grade, and depth is what keeps solids moving. Running a 4-inch building drain at the 2-inch minimum is legal, but it burns twice the fall for the same distance, which is exactly the problem when the street sewer is shallow and the run is long.
The other side is velocity. Too flat and flow drops below roughly 2 feet per second, solids settle and the line silts up. Too steep, above about 1/2 inch per foot, and liquid outruns the solids, strands them, and dries them into a blockage. This calculator computes velocity with Manning's equation at half-full flow, where the hydraulic radius is D/4, and flags both ends. Use it for field screening and layout; confirm final design against the code edition your AHJ has adopted, since the UPC and many state amendments are stricter than the IPC.
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Drain Pipe Slope Calculator FAQs
Under IPC Table 704.1 it depends on pipe size: 1/4 inch per foot (2.08%, 1:48) for 2-1/2 inches and smaller, 1/8 inch per foot (1.04%, 1:96) for 3 to 6 inches, and 1/16 inch per foot (0.52%, 1:192) for 8 inches and larger. The UPC is stricter, it starts everything at 1/4 inch per foot and only lets 4-inch and larger drop to 1/8 inch per foot with approval from the authority having jurisdiction.
At 1/4 inch per foot you need 25 inches, just over 2 feet. At 1/8 inch per foot you need 12.5 inches, and at 1/16 inch per foot you need 6.25 inches. Flip to the max run mode to work it backwards: enter the fall you actually have and the pipe size, and you get the longest run that fall covers at the code minimum.
Yes, though it is a practice limit rather than a code maximum. Above roughly 1/2 inch per foot (4.17%) the liquid runs ahead of the solids and leaves them behind in the pipe, where they dry out and block it. If you have to lose a lot of elevation, take it as a vertical drop with a proper fitting and then run the horizontal section at a normal grade rather than sloping the whole run steeply.
It is the flow speed generally accepted as enough to keep solids in suspension and scour the pipe invert clean. It is a design screening figure from UPC-lineage codes, state amendments and ASPE practice, not from IPC 704.1, whose own exception under Table 704.1 is the grease-interceptor rule requiring 1/4 inch per foot upstream of an interceptor. This calculator computes it with Manning's equation at half-full flow using a roughness coefficient set by the pipe material you select.
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