SizingKitTwo codes that disagree
Drain Pipe Slope Calculator
Enter a slope and a run and this free page returns the fall you have to build, then says whether that fall is legal — because the two model plumbing codes do not agree. The IPC allows 1/8 in per foot on 3 in through 6 in pipe; the UPC requires 1/4 in per foot on everything under 4 in, so an identical 3 in branch passes a rough-in in one jurisdiction and is torn out in the next. Both minimums are shown for every size, along with the velocity Manning's equation gives at the fall you entered. No signup, and the arithmetic runs in the browser.
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- IPC and UPC
- 1-1/4 in to 8 in
- Velocity at half full
Bores 3.068 in — Schedule 40 dimensions, which DWV PVC and ABS share.
Feet unless you write a unit. Measure along the pipe, not across the room.
Five ways of writing one measurement, and each field takes only its own trade's form: 4/12 typed into percent grade is refused rather than read as 0.33%.
Default 0.01 for pvc, abs, plastic; the published range is 0.009 to 0.011. n is empirical and dimensional — it is not a physical property and does not convert to a roughness height. The range within a material is as wide as the difference between materials, and joint quality, sags and debris move a real drain toward the top of its range. Use the design value for a new, well-laid line and the top of the range for anything existing.
This site carries schedule-pipe dimensions and does not carry hubless cast iron to CISPI 301, vitrified clay or corrugated culvert. Rather than substitute a neighboring standard, it asks: measure the socket or read the maker's submittal, and the velocity below is computed on what you type.
TOTAL FALL OVER THE RUN
5 in
5.00 in decimal · 127 mm
VELOCITY, RUNNING HALF FULL
2.42 ft/s
0.74 m/s · at or above the 2 ft/s scour figure
| Grade | 1.04% · 1 : 96 |
|---|---|
| IPC minimum, 3 in | 1/8" per ft |
| UPC minimum, 3 in | 1/4" per ft |
| Flow at half full | 28 gpm · 1.8 L/s |
| Bore the hydraulics used | 3.068 in · 77.9 mm |
| Fall per 10 ft | 1 1/4 in |
1/8" per ft meets the IPC minimum for 3 in pipe and does not meet the UPC's. This is the disagreement that fails a rough-in: the same 3 in line at this fall passes inspection in an IPC jurisdiction and gets torn out in a UPC one.
This is the IPC. The UPC is stricter and the difference is the single most common cause of a failed rough-in inspection on a 3 in line. UPC 708.0 requires horizontal drainage piping to run at not less than 1/4 in per foot throughout, and permits 1/8 in per foot only on pipe 4 in and larger, and then only where 1/4 in per foot is impractical and the authority approves it. A 3 in branch at 1/8 in per foot is compliant under the IPC and is not compliant under the UPC. Check which code the jurisdiction has adopted before sizing the fall.
What the fall buys: 3 in building drain capacity
IPC Table 710.1(2) rates a building drain by size and slope together, so fall is capacity you can sometimes dig for instead of paying for in pipe.
| Slope | Maximum drainage fixture units |
|---|---|
| 1/8 in per ft | 36 |
| 1/4 in per ft | 42 |
| 1/2 in per ft | 50 |
IPC (International Plumbing Code) 2021, Table 710.1(2) — Building Drains and Sewers: maximum number of drainage fixture units connected to any portion of the building drain or the building sewer, at slopes of 1/8, 1/4 and 1/2 in per foot.
The 1/16 in per foot column of the code table, which exists for 8 in and larger, and the 15 in row are omitted. A null cell means the code gives no entry — a 2 in or 2-1/2 in building drain may not be run at 1/8 in per foot at all. The governing footnote is not a number: any building drain that serves a water closet must be at least 3 in, whatever the fixture-unit arithmetic says, and in practice a residential building drain is 4 in. Under the UPC the 1/8 in per foot column is not available below 4 in in any case.
There is no maximum slope in IPC Table 704.1 or in UPC 708.0. The widely repeated rule that a drain steeper than 1/2 in per foot 'lets the water run away from the solids' is trade lore, not code, and it does not follow from the hydraulics: a steeper pipe carries a shallower, faster stream, and the transport of a solid depends on the depth of water around it rather than on the slope alone. Where a run genuinely has to drop a long way the answer is a vertical drop with a properly sloped run either side of it, not a uniformly steep line — and that is a detailing preference rather than a code requirement. What the code does constrain is the minimum, and the minimum is what a slope calculator should enforce.
Nominal pipe size (NPS) is a name, not a measurement. It matched the approximate inside diameter of wrought-iron pipe in the 1890s and has been a label ever since. A 3/4 in steel pipe measures 1.050 in outside and 0.824 in inside; 3/4 in is neither. From NPS 14 up the name finally equals the outside diameter, and below it the outside diameter is a fixed legacy value that every schedule of that size shares — the schedule changes the wall, which changes the inside diameter, never the outside. Copper, PEX and CPVC tube each run on a different naming convention again, so a 1/2 in copper tube, a 1/2 in PEX tube and a 1/2 in steel pipe have three different bores.
Recommended Standards for Wastewater Facilities (the Ten States Standards), which require sanitary sewers to be designed for a mean velocity of not less than 2.0 ft/s when flowing full. The same figure is the conventional self-cleansing velocity in building drainage practice. It is a design minimum for grit and solid transport in a sanitary sewer, carried across into building drainage by convention. Building drains rarely run at design flow, so the velocity at any given moment is usually below it; what matters is that the pipe reaches 2 ft/s during a fixture discharge, which is what the minimum slope is there to guarantee.
How to set the fall on a horizontal drain
Size, run, slope. The verdict line does the part that is not arithmetic.
Pick the size and measure the run
Measure along the pipe from the upstream invert to the downstream invert, not across the room — an offset adds developed length and therefore fall. The size drives everything else, because both codes set the minimum by diameter and because the hydraulics run on the bore behind the name: a 3 in Schedule 40 pipe is 3.068 in inside and a 4 in is 4.026 in.
Write the slope however your drawing writes it
Inches per foot, percent grade, a 1:48 ratio, a pitch out of 12, or degrees — five ways of writing the same measurement, and each field takes only its own trade's form. A 4/12 typed into the percent grade box is refused rather than read as 0.33%, because a plausible wrong answer is worse than a rejection.
Read the fall, then read the verdict
The plate gives total fall as a fraction a fitter can mark out and as millimeters, the fall per 10 ft for setting hangers, and the velocity at the half-full design condition. Under it, the verdict line names which of the two codes your slope satisfies. Check which one your jurisdiction has adopted before you cut anything — that is the single question this page exists to make unmissable.
What the two codes actually say
| IPC minimums | 1/4 in per foot (2%) on 2-1/2 in and smaller; 1/8 in per foot (1%) on 3 in through 6 in; 1/16 in per foot (0.5%) on 8 in and larger. Table 704.1 of the 2021 edition. |
|---|---|
| UPC minimum | 1/4 in per foot (2%) throughout, with 1/8 in per foot permitted on 4 in and larger only where 1/4 is impractical and the authority approves it. Section 708.0 of the 2021 edition. |
| Where they part company | 3 in pipe. Under the IPC a 3 in branch may fall 1/8 in per foot; under the UPC it may not, and the relief clause does not reach below 4 in. On 8 in pipe the gap is four to one — 1/16 against 1/4. |
| Maximum slope | There is none, in either code. The rule that a drain steeper than 1/2 in per foot outruns its solids is trade lore and is printed as such on this page rather than enforced. |
| Velocity method | Manning, V = (1.486/n) R^(2/3) S^(1/2), solved for a round pipe at a depth ratio of 0.5. A pipe running half full has the same hydraulic radius as one running full, D/4, so it moves at the same speed and carries exactly half the flow. |
| Roughness | Manning's n, editable, defaulting to 0.010 for plastic with the published 0.009–0.011 band shown. Cast iron, clay, concrete, brick and corrugated metal are in the list, each with its own range. |
| Scour figure | 2.0 ft/s, from the Ten States Standards for sanitary sewers. A recommendation rather than a code requirement, and flagged as one. |
| Capacity by slope | IPC Table 710.1(2): a 4 in building drain carries 180 fixture units at 1/8 in per foot and 250 at 1/2 in per foot — 39% more capacity bought with fall alone. |
Frequently asked questions
Is 1/8 in per foot enough for a 3 in drain?
It depends entirely on which code your jurisdiction adopted, and that is not a hedge — it is the actual state of the law. IPC Table 704.1 puts 3 in through 6 in pipe in the 1/8 in per foot band, so under the IPC a 3 in branch at that fall is inside the table. UPC 708.0 requires 1/4 in per foot and extends its 1/8 in relief only to pipe 4 in and larger, and then only with the authority's approval, so under the UPC the same branch is under the minimum. Most of the western United States is UPC territory and most of the east and midwest is IPC, but adoption is by state and often amended by county, so the answer is on your local amendment sheet and nowhere else.
Why is the minimum slope smaller for a bigger pipe?
Because depth of flow, not slope, is what carries a solid, and a larger pipe carries a deeper stream at the same fill fraction. Under Manning, velocity depends on the hydraulic radius to the two-thirds power, and hydraulic radius scales with diameter — so a 6 in drain at 1/8 in per foot moves its water about as fast as a 2 in drain at 1/4. The code minimums are not an arbitrary ladder; they are that relationship rounded to fractions a fitter can set with a level.
Can a drain be too steep?
Not according to either code, which set a minimum and no maximum at all. The widely repeated claim that water runs away from the solids above 1/2 in per foot does not follow from the hydraulics either: a steeper pipe carries a shallower, faster stream, and it is the depth of water around a solid rather than the slope of the pipe that moves it. Where a run genuinely has to drop a long way, the usual detailing answer is a vertical drop with properly graded pipe either side of it rather than one uniformly steep line — but that is a preference, and this page will not report it as a requirement.
What velocity should a building drain reach?
2 ft/s is the conventional scour figure, and it comes from the Ten States Standards for sanitary sewers rather than from the plumbing code. It matters that it is a design minimum at design flow, not a speed the pipe holds all day: a building drain spends most of its life empty and only reaches this velocity during a fixture discharge, which is precisely what the minimum slope exists to guarantee. Treat a result below 2 ft/s as a prompt to check the fall and the roughness, not as a failure.
Does the fall change how much the drain can carry?
Yes, and more than most people expect — it is the reason IPC Table 710.1(2) is indexed by size and slope together rather than by size alone. A 4 in building drain is rated for 180 drainage fixture units at 1/8 in per foot, 216 at 1/4 and 250 at 1/2. Where the site has the fall to spare, that is the cheapest capacity on the job; where it does not, it is the reason the drain goes up a size. The panel under the answer prints your size's three figures.
Why does the calculator want the bore rather than the pipe size?
Because Manning takes a diameter and the nominal size is not one. A 3 in DWV pipe bores 3.068 in and a 4 in bores 4.026 in, which is where the hydraulics on this page start; using 3 and 4 instead would report velocities a few percent low and flows several percent low. If your pipe is hubless cast iron to CISPI 301, vitrified clay or corrugated culvert, this site does not carry those dimensions and will not substitute a neighboring standard for them — measure the bore or read the submittal, and type it into the override field.
Is fall measured along the pipe or across the room?
Along the pipe. Slope is rise over run in the direction the pipe travels, so a line that jogs around a beam has more developed length than the straight-line distance and therefore more total fall. The practical consequence is at the ends: a 40 ft run at 1/4 in per foot drops 10 in, and if the connection at the far end was set from a plan dimension rather than a tape along the route, that is where the difference shows up as a flat spot.
Fall, and the two codes that disagree about it
There is no single American plumbing code. Two model codes are in force across the country — the International Plumbing Code and the Uniform Plumbing Code — and on drainage slope they say different things about the same pipe. The IPC grades its minimum by size, allowing bigger pipe to run flatter because bigger pipe carries a deeper stream: 1/4 in per foot up to 2-1/2 in, 1/8 in from 3 in to 6 in, 1/16 in above that. The UPC takes the simpler and stricter line: 1/4 in per foot on everything, with 1/8 in available on 4 in and up only where the steeper fall is impractical and the authority having jurisdiction signs off on it. On a 3 in line the two codes give incompatible answers, and since a 3 in branch serving a water closet is one of the most common runs in residential work, that disagreement fails more rough-ins than any other single item in this section.
The other thing worth unlearning is the maximum. Almost every discussion of drain slope eventually produces the claim that too much fall lets the water outrun the solids and leaves a blockage behind — usually with 1/2 in per foot named as the ceiling. It is worth being precise about what that is: it is something plumbers say, and it is not in IPC Table 704.1 or in UPC 708.0, neither of which sets any maximum at all. The physical story behind it is also weaker than it sounds. A steeper pipe does carry a shallower and faster stream, but solid transport depends on the depth of water surrounding the solid rather than on the pipe's angle, and a steep, well-flushed line is not obviously worse than a flat one at the minimum. Where a drain has to lose a lot of height, the detailing answer is a vertical drop with graded runs either side — a preference this page will describe and will not enforce.
What the calculator adds beyond the fall itself is the consequence. Manning's equation gives the velocity and the flow at the half-full condition a building drain is designed around, using the real bore rather than the nominal size — the same distinction the pipe volume calculator is built on. And because IPC Table 710.1(2) rates a building drain by size and slope together, the panel below the answer shows what your size carries at each of the three code slopes, which is how you find out whether digging deeper is cheaper than going up a pipe size. If the run ends at a tank rather than a sewer, the septic tank size calculator picks up where this page stops, and if it has to be lifted rather than fallen, the pump head calculator does.
What happens to the dimensions you type
Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type — loads, lengths, nameplate ratings, the rates your utility charges you — is uploaded, logged or kept, which is also why the calculators carry on working in a mechanical room with no signal.
The IPC slope bands, the UPC section and the fixture-unit capacity table are compiled into the page, so the verdict line is decided in the tab with no lookup and no request.