Drain Pipe Size Calculator — IPC Table 710.1
Drainage sizing is a table lookup with one important override. Enter the DFU load and what you are sizing — a building drain or sewer, a horizontal fixture branch, or a stack — and this returns the smallest pipe the code allows, the capacity you are left with, and the minimum fall it needs. If a water closet is on the run, the 3-inch floor is applied and flagged.
Size your drain
Building drains use Table 710.1(1); branches and stacks use Table 710.1(2).
Applies to building drains and sewers only.
Floors a building drain at 3 inches.
Used to work out the total fall the run needs.
Minimum drain size
The DFU load alone would ride 2", but a building drain serving a water closet is floored at 3 inches.
That size carries
42 DFU
Spare capacity
24 DFU
The water-closet floor applied
The footnote to Table 710.1(1) sets a 3-inch minimum on any building drain serving a water closet, whatever the DFU total says. Turn the toggle off to see the load-only answer.
See the breakdown
Planning estimate. Water-closet count limits in the Table 710.1(2) footnotes (two on a 3-inch branch, six on a 3-inch stack) are not applied automatically — check them. A licensed plumber and your AHJ have final say.
The method, explained in plain English
Two tables, chosen by what you are sizing, plus the rules that sit on top of them.
Why stacks are rated by height
Water falling in a stack reaches terminal velocity within a couple of storeys and settles into an annular sheet on the wall with an air core down the middle. That regime moves more than unsteady falling water, so a stack over three branch intervals gets a higher rating.
Steeper is not always better
Above about 1/2 inch per foot the liquid can outrun the solids and leave them behind. Drainage wants a self-scouring flow, roughly 2 ft/s, not the fastest possible one.
Oversizing causes blockages
A too-large pipe at minimum grade runs shallow and slow, and solids settle. This is the counter-intuitive failure mode behind a lot of 4-inch house drains that should have been 3.
The branch interval
A vertical length of stack at least 8 feet high taking the branches of one storey. It is why the stack columns split at three, and why there is a separate cap on discharge into any single interval.
Worked examples
The same 18 DFU in three places, and what the slope choice buys on a bigger sewer.
House building drain — 18 DFU, 1/4 in/ft
water closet on the run → floor at 3"
→ 3 inch, capacity 42 DFU, 24 spare
min slope for 3" = 1/8 in/ft, so 1/4 is comfortably compliant
fall over 40 ft at 1/4 = 10.0 in
Result: 3 inch. Note the calculator is run at 1/4 in/ft here even though 3-inch only requires 1/8 — extra fall is fine and helps scouring, it is too little that causes trouble.
The same 18 DFU as a branch and as a stack
stack, ≤3 branch intervals: 2" = 10, 2-1/2" = 20 → 2-1/2 inch
stack, >3 branch intervals: 2" = 24 → 2 inch
Result: three different answers for one load. The tall stack is the smallest — a 2-inch pipe carries 10 DFU over three storeys but 24 over more than three, because terminal-velocity annular flow only develops with height.
Apartment block sewer — what slope buys
200 DFU on a building sewer, three slope options.
at 1/4 in/ft: 4" carries 216 ✓ → 4 inch
at 1/2 in/ft: 4" carries 250 ✓ → 4 inch
Result: going from 1/8 to 1/4 inch per foot saves a whole pipe size. Over a long sewer run that is a meaningful saving in pipe and trench width — as long as the fall is available, since 1/4 in/ft over 100 feet needs 25 inches of depth.
IPC Table 710.1(1) — building drains and sewers
Maximum DFU load at each slope, rendered from the same data the calculator uses. A dash means the size is not listed at that slope. The last column is the minimum slope the size itself requires under Table 704.1.
| Size | 1/16 in/ft | 1/8 in/ft | 1/4 in/ft | 1/2 in/ft | Min slope |
|---|---|---|---|---|---|
| 1-1/4" | — | — | 1 | 1 | 0.25 in/ft |
| 1-1/2" | — | — | 3 | 3 | 0.25 in/ft |
| 2" | — | — | 21 | 26 | 0.25 in/ft |
| 2-1/2" | — | — | 24 | 31 | 0.25 in/ft |
| 3" | — | 36 | 42 | 50 | 0.125 in/ft |
| 4" | — | 180 | 216 | 250 | 0.125 in/ft |
| 5" | — | 390 | 480 | 575 | 0.125 in/ft |
| 6" | — | 700 | 840 | 1000 | 0.125 in/ft |
| 8" | 1400 | 1600 | 1920 | 2300 | 0.0625 in/ft |
| 10" | 2500 | 2900 | 3500 | 4200 | 0.0625 in/ft |
| 12" | 3900 | 4600 | 5600 | 6700 | 0.0625 in/ft |
| 15" | 7000 | 8300 | 10000 | 12000 | 0.0625 in/ft |
IPC Table 710.1(2) — horizontal branches and stacks
Slope-independent. Compare the last two columns and watch the tall-stack figures overtake the short ones at every size — that is terminal velocity doing the work.
| Size | Horizontal branch | One branch interval | Stack, ≤3 intervals | Stack, >3 intervals |
|---|---|---|---|---|
| 1-1/2" | 3 | 2 | 4 | 8 |
| 2" | 6 | 6 | 10 | 24 |
| 2-1/2" | 12 | 9 | 20 | 42 |
| 3" | 20 | 20 | 48 | 72 |
| 4" | 160 | 90 | 240 | 500 |
| 5" | 360 | 200 | 540 | 1100 |
| 6" | 620 | 350 | 960 | 1900 |
| 8" | 1400 | 600 | 2200 | 3600 |
| 10" | 2500 | 1000 | 3800 | 5600 |
| 12" | 3900 | 1500 | 6000 | 8400 |
| 15" | 7000 | see footnote | see footnote | see footnote |
Sources & standards: IPC 2021 Table 710.1(1) (building drains and sewers), Table 710.1(2) (horizontal fixture branches and stacks) with its water-closet count footnotes, Table 704.1 (minimum slope), and Table 709.1 (drainage fixture unit values). Roughly fifteen states use the UPC or a derivative, which sizes drainage differently. Local amendments override the model code.
Frequently asked questions
Common questions about drain sizing, stacks, slope, and the water-closet rule.
What size drain pipe do I need?
For a typical house at 18 DFU on a building drain at 1/4 inch per foot, the answer is 3 inch — and not because of the load. Table 710.1(1) would carry 18 DFU on a 2-inch drain, but a water closet anywhere on a building drain floors it at 3 inches. The 3-inch pipe then carries 42 DFU, well over double what the house produces.
Why does a taller stack carry more than a short one?
This is the most counter-intuitive line in the whole table, and almost nobody explains it. A 2-inch stack carries 10 DFU over three branch intervals or less, but 24 DFU over more than three. The reason is terminal velocity: water falling in a stack accelerates until friction against the pipe wall balances gravity, then settles into a stable annular sheet clinging to the wall with air down the middle. That regime only develops with height — a short stack never reaches it, so it is rated more conservatively.
Does slope change the size I need?
For a building drain or sewer, yes — steeper carries more. A 4-inch building drain takes 180 DFU at 1/8 in/ft, 216 at 1/4, and 250 at 1/2. For horizontal branches and stacks, no: Table 710.1(2) is slope-independent, because those runs are already assumed to be at the fall the code requires.
Can I put a toilet on a 2-inch drain?
No. Independently of DFU load, a water closet needs a 3-inch branch, and any building drain serving one is floored at 3 inches by the footnote to Table 710.1(1). The load arithmetic will happily tell you 2 inch is enough — it is wrong, and this is the mistake the calculator is built to catch.
How many toilets can I put on a 3-inch pipe?
The DFU capacity is not the binding limit here — the footnotes to Table 710.1(2) are. A 3-inch horizontal branch is limited to two water closets, and a 3-inch stack to six, regardless of what the DFU column allows. Check the footnotes rather than the number whenever water closets are involved.
What is a branch interval?
A vertical length of stack, at least 8 feet high, within which the horizontal branches from one storey connect. In practice it is one floor of a building. It matters because the stack columns in Table 710.1(2) split at three intervals, and because there is a separate limit on how much can discharge into any single interval.
Should I oversize the drain to be safe?
Not much, and never at the expense of slope. A drain flowing too slowly does not carry solids — the liquid outruns them and they settle. An oversized pipe at minimum grade runs shallower and slower than a correctly sized one, which is how a 4-inch line in a house that needs 3-inch ends up blocking. Size it right and get the fall right.
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