Drain Pipe Size Chart — Maximum DFU by Size & Slope

Maximum drainage fixture units for every drain, sewer, branch, and stack size — the tables you read after totaling DFU on the fixture-unit chart. IPC and UPC are kept separate because their capacities differ for the same pipe; read from the code family your jurisdiction enforces and check its local amendments. IPC values per Tables 710.1(1) and 710.1(2) (2021 edition; unchanged 2012–2024); UPC values per Table 703.2 (2021 edition).

IPC — Building drains & sewers, by slope

IPC Table 710.1(1)

Maximum DFU connected to any portion of the building drain or building sewer, including building-drain branches, by pipe diameter and slope per foot. Values per IPC Table 710.1(1) (2021 edition). A dash means that size is not permitted at that slope.
Size	1/16"/ft	1/8"/ft	1/4"/ft	1/2"/ft
1-1/4"	—	—	1	1
1-1/2"	—	—	3	3
2"	—	—	21	26
2-1/2"	—	—	24	31
3"	—	36	42	50
4"	—	180	216	250
5"	—	390	480	575
6"	—	700	840	1,000
8"	1,400	1,600	1,920	2,300
10"	2,500	2,900	3,500	4,200
12"	3,900	4,600	5,600	6,700
15"	7,000	8,300	10,000	12,000

The highlighted 1/4"/ft column is the standard slope most jobs run. A building drain serving any water closet is 3" minimum no matter what the DFU total allows. This 1/4"/ft column is the same data behind our fixture unit calculator's drain-size result.

IPC — Horizontal fixture branches & stacks

IPC Table 710.1(2)

Maximum DFU on a horizontal fixture branch, discharging into one branch interval, and on drainage stacks by height class. Values per IPC Table 710.1(2) (2021 edition). Does not apply to branches of the building drain — size those from the table above.
Size	Horiz. branch	One 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	1,100
6"	620	350	960	1,900
8"	1,400	600	2,200	3,600
10"	2,500	1,000	3,800	5,600
12"	3,900	1,500	6,000	8,400
15"	7,000	—	—	—

Dashes in the 15" row: stacks that large are sized by design criteria, not the table. A stack may shrink as accumulated load decreases going up, but never below half the diameter of the largest required stack size.

UPC — Drainage piping, vertical & horizontal

UPC Table 703.2

Maximum DFU on vertical and horizontal drainage piping (building drains and sewers included) and maximum vertical developed length, per the drainage rows of UPC Table 703.2 (2021 edition). Horizontal capacities assume 1/4" per foot slope; note numbers are explained below.
Size	Vertical DFU	Horiz. DFU	Vert. max length (ft)	Notes
1-1/4"	1	1	45	—
1-1/2"	2	1	65	2, 7
2"	16	8	85	3
3"	48	35	212	4
4"	256	216	300	5
5"	600	428	390	5
6"	1,380	720	510	5
8"	3,600	2,640	750	5
10"	5,600	4,680	—	5
12"	8,400	8,200	—	5

Horizontal run length is unlimited under the UPC — only vertical length is capped. At 1/8" per foot slope, multiply the horizontal DFU capacity by 0.8 (note 5); 1/8" per foot is only an option for 4" and larger piping, with approval.

UPC table notes

1The DFU loads in the drainage rows exclude the trap arm.
2The 2-DFU allowance on 1-1/2" vertical piping does not extend to sinks, urinals, or dishwashers exceeding 1 fixture unit.
3No water closets or six-unit traps on 2" drainage piping.
4At most five water closets or five six-unit traps on 3" piping (the 2024 cycle raises the water-closet count to six).
5Horizontal capacities assume 1/4" per foot slope; at 1/8" per foot, multiply the horizontal fixture-unit capacity by 0.8.
7Up to 8 public lavatories may share a 1-1/2" vertical branch or a 1-1/2" horizontal branch at 1/4" per foot (2021 addition).

How to read this chart

Total the drainage fixture units for everything upstream — the DFU chart has the per-fixture values, and the fixture unit calculator runs the count — then find the smallest pipe size whose limit covers the total. Under the IPC, which table depends on what you're sizing: building drains and sewers read from the slope matrix, while fixture branches and stacks read from the second table (a stack "branch interval" is roughly a story's worth of stack between branch connections). Under the UPC, pick the vertical or horizontal column.

A building drain that serves a water closet must be at least 3" no matter what the DFU total would otherwise allow. Under the UPC, no water closets or six-unit traps on 2" drainage piping. Water-closet counts, not raw DFU, are often what actually set the size on small sanitary lines.

Slope is part of the answer

A drain's capacity is a function of its slope, and below a minimum slope a size simply isn't permitted — that's what the dashes in the IPC matrix encode: nothing smaller than 3" may run at 1/8" per foot, and nothing smaller than 8" at 1/16" per foot. The pipe slope calculator converts between slope ratios, percent grade, and fall over a run.

IPC and UPC do not agree — and local amendments override both

The same 6" horizontal drain carries 840 DFU under the IPC at 1/4" per foot but 720 under the UPC; an 8" carries 1,920 under the IPC and 2,640 under the UPC. A DFU total counted from one code's fixture values must be sized against that same code's pipe tables — the two systems only make sense as matched pairs. Both tables here are the model codes: your state or city may amend them, so confirm the enforced edition and its amendments before finalizing a design.

These tables are stable ground: the IPC 710.1 tables are value-identical from the 2012 through 2024 editions, and the UPC 703.2 numbers are unchanged from 2015 through the 2024 cycle (its footnotes did evolve — the 2021 edition rewrote the 3" water-closet limit and added the public-lavatory allowance on 1-1/2" piping).

Common questions

How many fixture units can a 4 inch drain handle?

Under the IPC a 4" building drain or sewer carries up to 216 DFU at the standard 1/4" per foot slope (180 at 1/8", 250 at 1/2"); as a horizontal fixture branch it carries 160 DFU, and as a stack 240–500 DFU depending on branch intervals. Under the UPC a 4" drain carries 216 DFU horizontal at the 1/4" per foot basis and 256 DFU vertical.

How many fixture units can a 3 inch drain handle?

IPC: 42 DFU as a building drain at 1/4" per foot (36 at 1/8", 50 at 1/2"), 20 DFU as a horizontal branch, and 48–72 DFU as a stack. UPC: 35 DFU horizontal, 48 DFU vertical — with at most five water closets or five six-unit traps on any 3" line (2021 edition). Both codes cap water-closet counts well before the raw DFU number runs out.

What is the minimum drain size for a toilet?

Under the IPC, any building drain serving a water closet must be at least 3" regardless of the DFU math. Under the UPC, water closets are not permitted on 2" drainage piping at all, and a 3" line is limited to five water closets (raised to six in the 2024 cycle) — so 3" is the practical floor under both codes.

Does slope change how many DFU a drain can carry?

Yes. Flatter pipe carries less: the IPC tabulates each slope separately (a 6" building drain drops from 840 DFU at 1/4" per foot to 700 at 1/8"), and the UPC applies a 0.8 multiplier to its horizontal capacities at 1/8" per foot. Below a size's minimum slope there is no capacity at all — the IPC prints no value for anything under 3" below 1/4" per foot, or under 8" below 1/8" per foot.

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