IPC Table 704.1 against UPC 708.1, and they disagree at 3 inches
A 3 inch drain needs 1/8 inch of fall per foot under the International Plumbing Code and 1/4 inch per foot under the Uniform Plumbing Code. Same pipe, same toilet, twice the fall, depending only on which model code your state adopted. Over a 30 foot run that is 3 3/4 inches against 7 1/2, and on a basement bathroom that difference is often the whole question of whether it drains by gravity or needs a pump.
Minimum slope and fall, under the code your state uses
ft
in
Which model code applies is set by your state and sometimes amended by your city. Ask the building department before the plumbing is laid out, because it changes how far a fixture can sit from the drain.
Specification summary
Generated from on .
Reopen that address to reproduce these figures exactly.
Entered
Result
One run, at each code's minimum slope
Worked example
A 3 inch drain running 30 feet to the building drain, laid out first under the IPC and then under the UPC.
IPC. Table 704.1 puts 3 in pipe in the 3 to 6 in band: 1/8 in per ft.
Fall. 1/8 × 30 = 3.75 in, which is 1.04%.
UPC. 708.1 sets 1/4 in per ft, and the exception starts at 4 in: 1/4 in per ft.
Fall. 1/4 × 30 = 7.5 in, which is 2.08%.
With 6 in of drop available. Enough under the IPC, short by 1.5 in under the UPC.
Longest UPC run on 6 in. 6 ÷ 1/4 = 24 ft.
Same pipe, same fixture, same basement. Under one code the bathroom drains by gravity with more than two inches to spare. Under the other it runs out of fall six feet before the drain, and the fix is either moving the bathroom or fitting a sewage ejector. That is why the model code in force is a layout question and not an inspection detail.
The formula
Slope is per foot. Fall is per run. The code only gives you the first:
fall = slope × run percent = slope ÷ 12 × 100 max run = drop available ÷ slope
slope
inches of drop per foot of horizontal run, the minimum from the code in force
1/4 in/ft
2.08 per cent
1/8 in/ft
1.04 per cent
1/16 in/ft
0.52 per cent, IPC only, 8 in pipe and larger
run
horizontal length of the drain, not the length of pipe
The last equation is the useful one on a slab or in a basement. The drop available is fixed by where the building drain leaves the house, so dividing it by the slope tells you how far from the drain a fixture can be before gravity stops working.
Why this page asks which code you are on
Drain slope calculators give you one answer per pipe size, and the answer they give is usually the IPC table, printed without saying so.
That is correct in roughly half the country. The other half adopted the Uniform Plumbing Code, which does not have a table at all. It has a rule of 1/4 inch per foot for every horizontal drain, and an exception that lets 4 inch pipe and larger drop to 1/8, but only where 1/4 is impractical and only where the building official has approved it first.
So a 3 inch pipe, which is the size most house drains are, sits in the IPC's 1/8 band and outside the UPC's exception entirely. The two codes require different slopes for the commonest drain in a house.
The difference is a factor of two in fall, and on a run to a basement or slab drain that is frequently the difference between gravity and a pump. It is not a detail to discover at inspection.
Minimum drain slope by pipe size, IPC against UPC
Pipe size
IPC Table 704.1
UPC 708.1
The difference
Fall over 30 ft, IPC against UPC
1.25 in
1/4 in/ft (2.08%)
1/4 in/ft
same
7.5 in against 7.5 in
1.5 in
1/4 in/ft (2.08%)
1/4 in/ft
same
7.5 in against 7.5 in
2 in
1/4 in/ft (2.08%)
1/4 in/ft
same
7.5 in against 7.5 in
2.5 in
1/4 in/ft (2.08%)
1/4 in/ft
same
7.5 in against 7.5 in
3 in
1/8 in/ft (1.04%)
1/4 in/ft
IPC is 1/2 of the fall
3.75 in against 7.5 in
4 in
1/8 in/ft (1.04%)
1/4 in/ft, 1/8 with approval
IPC is 1/2 of the fall
3.75 in against 7.5 in
6 in
1/8 in/ft (1.04%)
1/4 in/ft, 1/8 with approval
IPC is 1/2 of the fall
3.75 in against 7.5 in
8 in
1/16 in/ft (0.52%)
1/4 in/ft, 1/8 with approval
IPC is 1/4 of the fall
1.875 in against 7.5 in
Read 2026-09-13. Slopes quoted from IPC Table 704.1 as adopted in Tennessee 2021 and UPC 708.1 as adopted in Hawaii 2021. The fall column is this page multiplying.
Two model codes, and they disagree about the commonest drain
American plumbing runs on one of two model codes depending on the state, and the minimum slope of a horizontal drain is one of the places they part company.
The International Plumbing Code puts it in a table:
Horizontal drainage piping shall be installed in uniform alignment at uniform slopes. The slope of a horizontal drainage pipe shall be not less than that indicated in Table 704.1 except that where the drainage piping is upstream of a grease interceptor, the slope of the piping shall be not less than 1/4 inch per foot (2-percent slope).
The Uniform Plumbing Code has no table. It has a rule and an exception:
Horizontal drainage piping shall be run in practical alignment and a uniform slope of not less than 1/4 inch per foot (20.8 mm/m) or 2 percent toward the point of disposal provided that, where it is impractical due to the depth of the street sewer, to the structural features, or to the arrangement of a building or structure to obtain a slope of 1/4 inch per foot (20.8 mm/m) or 2 percent, such pipe or piping 4 inches (100 mm) or larger in diameter shall be permitted to have a slope of not less than 1/8 inch per foot (10.4 mm/m) or 1 percent, where first approved by the Authority Having Jurisdiction.
Minimum slope by pipe size under each code, as adopted in Tennessee and Hawaii respectively, 2021 editions
Pipe size
IPC Table 704.1
UPC 708.1
The difference
Fall over 30 ft, IPC against UPC
1.25 in
1/4 in/ft (2.08%)
1/4 in/ft
same
7.5 in against 7.5 in
1.5 in
1/4 in/ft (2.08%)
1/4 in/ft
same
7.5 in against 7.5 in
2 in
1/4 in/ft (2.08%)
1/4 in/ft
same
7.5 in against 7.5 in
2.5 in
1/4 in/ft (2.08%)
1/4 in/ft
same
7.5 in against 7.5 in
3 in
1/8 in/ft (1.04%)
1/4 in/ft
IPC is 1/2 of the fall
3.75 in against 7.5 in
4 in
1/8 in/ft (1.04%)
1/4 in/ft, 1/8 with approval
IPC is 1/2 of the fall
3.75 in against 7.5 in
6 in
1/8 in/ft (1.04%)
1/4 in/ft, 1/8 with approval
IPC is 1/2 of the fall
3.75 in against 7.5 in
8 in
1/16 in/ft (0.52%)
1/4 in/ft, 1/8 with approval
IPC is 1/4 of the fall
1.875 in against 7.5 in
Read the 3 inch row. Under the IPC it drops to 1/8 inch per foot as of right. Under the UPC the exception does not start until 4 inches, so a 3 inch drain stays at 1/4. Twice the fall, for the same pipe carrying the same toilet.
And the UPC exception is narrower than a table entry even where it applies. It needs 1/4 to be impractical, for one of three stated reasons, and it needs the Authority Having Jurisdiction to have approved it first. It is a variance written into the code, not a size band.
Why a factor of two decides whether a basement drains
Slope is a small number per foot and a large number per run. At 1/4 inch per foot, 30 feet of drain falls 7 1/2 inches. At 1/8 it falls 3 3/4.
On a slab or a basement floor that difference is not cosmetic. The building drain has to leave the house at a fixed height, set by the sewer or the septic tank, and every foot of horizontal run towards it eats into the depth you have. A fixture far from the stack, on a code that insists on 1/4, can run out of fall before it reaches the drain, and the answer at that point is a sewage ejector pump rather than a pipe.
Which is why the code in force matters before the plumbing is laid out, not when it is inspected. The calculator runs both codes on the same run so the difference is visible, and asks which one your jurisdiction adopted.
Percent and inches per foot are the same thing. 1/4 inch per foot is 2.08 per cent, which both codes round to 2 in their own metric conversions, and 1/8 is 1.04, rounded to 1. Levels and lasers read in either, so the calculator prints both.
Measuring fall on site, and what uniform means
Both codes require a uniform slope, not merely an average one. A drain that falls correctly from end to end but sags in the middle holds water in the sag, and what settles there stays.
A level and a block are enough. A 4 ft level with a 1 in block under its low end reads 1/4 in per ft when the bubble is centred; a 1/2 in block reads 1/8.
For a long run, set it from the ends and check the middle. Fix the start and end heights from the fall this page gives you, pull a string line between them, and hang the pipe to the string. Checking every few feet catches a bracket that has sagged.
Support spacing matters more for plastic than cast iron. A drain that was uniform on installation day can sag between hangers as it warms, which is why the hanger spacing in the code in force is part of keeping the slope, not a separate rule.
When there is not enough fall
If the calculator says the run is short, the geometry has not got a clever answer. There are four real ones.
Shorten the run. Moving the fixture closer to the stack or the building drain removes fall requirement foot for foot.
Lower the drain. On new work the building drain can often leave deeper; on existing work that means breaking out a slab.
Go up a size, under the IPC. Moving from 2 1/2 in to 3 in pipe halves the required slope in the IPC table. It does nothing under the UPC until 4 in, and even then only with approval.
Pump it. A sewage ejector or a macerating unit lifts waste to a drain that is higher than the fixture. It works, it needs power and maintenance, and it is the answer basement bathrooms end up with more often than people expect.
Slope, grade, and the upstream of a grease interceptor
The two codes do not use the same word. The IPC section is titled slope of horizontal drainage piping. The UPC section is titled grade of horizontal drainage piping. It is the same quantity, and the heading is a quick tell for which model code a specification or an inspection report was written against.
The IPC has one exception that runs the other way. Piping upstream of a grease interceptor goes to 1/4 inch per foot whatever its size, because grease that settles in a shallow run before the interceptor never reaches it. That is a commercial kitchen provision and it rarely touches a house, but it is written into the same sentence as the table.
Neither section sets a maximum. The belief that a drain can be too steep, that the water outruns the solids, is widely repeated and is not in either section read here. What the codes care about is a minimum and a uniform slope: a run that sags or bellies holds water whatever its average fall.
The sources, and why two states
Read 2026-09-13.
Tennessee Plumbing Code 2021, Section 704.1 and Table 704.1, International Plumbing Code as adopted by Tennessee, 2021 edition, Chapter 7 Sanitary Drainage, read through UpCodes with the jurisdiction and edition stated on the page. Sets minimum slope by pipe size in a table: 1/4 inch per foot for 2 1/2 inches or less, 1/8 for 3 to 6 inches, 1/16 for 8 inches and larger.
Hawaii Plumbing Code 2021, Section 708.1 Grade of Horizontal Drainage Piping, Uniform Plumbing Code as adopted by Hawaii, 2021 edition, Chapter 7 Sanitary Drainage, read through UpCodes with the jurisdiction and edition stated on the page. Sets 1/4 inch per foot for all horizontal drainage piping, permitting 1/8 only for 4 inch pipe and larger, only where 1/4 is impractical, and only where first approved by the Authority Having Jurisdiction.
Both model plumbing codes are paywalled and codes.iccsafe.org returns HTTP 403 to this site, so both are read here as a state's adopted text. UpCodes is known to serve whichever jurisdiction it chooses when a page is fetched without one, which is a trap this site has walked into before, so both pages were checked for the jurisdiction and edition they actually displayed: Tennessee 2021 for the IPC table and Hawaii 2021 for the UPC rule.
Your state may have adopted either, or amended either. A handful of states write their own plumbing code, and cities amend. The building department will tell you which applies; it is the first question, not the last.
Frequently asked questions
What is the minimum slope for a drain pipe?
It depends on the model code your state adopted. Under IPC Table 704.1 it is 1/4 in per ft for pipe 2 1/2 in or smaller, 1/8 in per ft for 3 to 6 in, and 1/16 in per ft for 8 in and larger. Under UPC 708.1 it is 1/4 in per ft for everything, with 4 in pipe and larger permitted to go to 1/8 only where 1/4 is impractical and the building official has approved it first.
What slope does a 3 inch drain need?
1/8 in per ft under the IPC and 1/4 in per ft under the UPC. It is the size where the two codes part company: the IPC's 1/8 band starts at 3 in, and the UPC's exception does not start until 4 in.
How much fall does a 20 foot drain need?
At 1/4 in per ft, 5 in. At 1/8 in per ft, 2 1/2 in. Multiply the slope by the horizontal run in feet. Which slope applies depends on the pipe size and on which model plumbing code your state uses.
Is 1/4 inch per foot the same as 2 percent?
Very nearly. 1/4 in per ft is 2.08 per cent and 1/8 in per ft is 1.04 per cent, so 2 and 1 per cent are close approximations but slightly shallower than the fractions.
Can a drain pipe have too much slope?
Neither of the two code sections read for this page sets a maximum slope. What both require is a minimum and a uniform slope, and a run that sags between hangers is the failure a uniform slope rules out.
What is the difference between slope and grade in plumbing?
Nothing but the code that uses the word. The IPC section is titled slope of horizontal drainage piping and the UPC section is titled grade of horizontal drainage piping. The heading is a quick tell for which model code a specification was written against.
What if I do not have enough fall for a basement bathroom?
Shorten the run to the drain, lower the drain, or under the IPC move up a pipe size, which halves the slope between 2 1/2 and 3 in. Failing those, a sewage ejector or macerating unit pumps the waste up to a higher drain.
Which plumbing code does my state use?
States adopt either the International Plumbing Code or the Uniform Plumbing Code, some write their own, and many cities amend what their state adopted. The building department will tell you which applies, and it is worth asking before laying out the drains.
Check these numbers yourself
Tennessee Plumbing Code 2021, Section 704.1 and Table 704.1International Plumbing Code as adopted by Tennessee, 2021 edition, Chapter 7 Sanitary Drainage, read 2026-09-13 through UpCodes with the jurisdiction and edition confirmed on the page. Minimum slope 1/4 in per ft for pipe 2 1/2 in or less, 1/8 for 3 to 6 in, 1/16 for 8 in and larger, and 1/4 in per ft for piping upstream of a grease interceptor.
Hawaii Plumbing Code 2021, Section 708.1 Grade of Horizontal Drainage PipingUniform Plumbing Code as adopted by Hawaii, 2021 edition, Chapter 7 Sanitary Drainage, read 2026-09-13 through UpCodes with the jurisdiction and edition confirmed on the page. Minimum 1/4 in per ft for all horizontal drainage piping, with 4 in and larger permitted to have 1/8 only where 1/4 is impractical and where first approved by the Authority Having Jurisdiction.
How every figure here is verified: Sources & Method. Who builds this: About. Found something wrong? Tell us and it gets fixed or removed.
Figures on this page last checked against the source documents on 2026-09-13. Codes are amended locally; confirm against the edition your jurisdiction enforces.
The slopes are quoted from one state's adoption of each model code, Tennessee 2021 for the IPC and Hawaii 2021 for the UPC, read on 13 September 2026. Your state may have adopted either code, a different edition, or its own amendments, and your city may amend further. Both model codes are paywalled and were read here in their state-adopted form. The UPC exception requires approval from the building official, which this page cannot give. Fixture unit sizing, vents and hanger spacing are not calculated here.