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Plumbing

DFU & Drain Size Calculator

Count drainage fixture units and size the horizontal branch, stack, and building drain by slope — with the IPC 2021 tables and the water-closet rules shown.

Fixtures
Bathroom group (WC + lav + tub/shower)
5 DFU
Water closet (1.6/1.28 gpf)
3 DFU
Lavatory (bathroom sink)
1 DFU
Shower stall
2 DFU
Bathtub (with shower)
2 DFU
Kitchen sink (+ disposal / DW on same trap)
2 DFU
Dishwasher (separate)
2 DFU
Clothes washer standpipe
2 DFU
Laundry tray / utility sink
2 DFU
Bar / prep sink
1 DFU
Floor drain (2")
2 DFU
Building-drain slope

Flatter slopes carry fewer fixture units.

Building drain
1/4″ per ft
3"min. drain
Total DFU
7
Branch
3"
Stack
3"
Building drain
3"
Fixture tally (2)
Bathroom group (WC + lav + tub/shower)1 × 55 DFU
Kitchen sink (+ disposal / DW on same trap)1 × 22 DFU
ƒShow the math
  1. 1.Total drainage fixture unitsper IPC 2021 Table 709.1
    Σ qty × DFU per fixture
    1×5 + 1×2
    = 7 DFU
  2. 2.Horizontal fixture branchper IPC 2021 Table 710.1(1)

    Smallest branch with capacity ≥ 7 DFU, floored to 3" because a water closet is present.

    = 3"
  3. 3.Drainage stack (≤ 3 branch intervals)per IPC 2021 Table 710.1(1)

    Smallest stack with capacity ≥ 7 DFU, 3" minimum for the water closet.

    = 3"
  4. 4.Building drain @ 1/4″ per ftper IPC 2021 Table 710.1(2)

    Smallest building drain with capacity ≥ 7 DFU at 1/4″/ft slope, 3" minimum for the water closet.

    = 3"
Before you rely on this
  • A water closet requires a 3-inch minimum drain — a 2-inch branch can carry the fixture-unit count but not a toilet.
  • Never reduce a drain in the direction of flow — size to the largest requirement and carry it downstream.
  • Sizing uses IPC 2021 private-use (residential) fixture units. Values differ under the UPC — check which code your state adopts.

How the drain sizing works

Every fixture carries a drainage-fixture-unit (DFU) rating from IPC 2021 Table 709.1. The tool adds qty × DFU across your fixtures, then reads the smallest pipe whose capacity meets that total in three contexts:

  • Horizontal fixture branch — Table 710.1(1). A 2-inch branch carries 6 DFU, 3-inch carries 20.
  • Drainage stack — Table 710.1(1), for a stack of three branch intervals or fewer.
  • Building drain — Table 710.1(2), where capacity rises with slope (1/8, 1/4, or 1/2 inch per foot).

Two hard rules override the DFU math: any water closet forces a 3-inch minimum drain, and a 3-inch horizontal branch is capped at two water closets. Both are applied automatically and shown in the math. A drain is never reduced in the direction of flow — size to the largest requirement and carry it downstream.

Frequently asked

What is a drainage fixture unit (DFU)?
A drainage fixture unit is a rating that stands in for the discharge load of a plumbing fixture, so different fixtures can be added on one scale. In IPC 2021 Table 709.1 a private lavatory is 1 DFU, a shower is 2, a 1.6-gpf water closet is 3, and a full bathroom group is 5. You total the DFUs on a pipe, then read the smallest size whose capacity meets or exceeds that total.
What size drain pipe do I need for a house?
Total the DFUs of everything draining to the pipe, then size each context from IPC Table 710.1. A typical 2.5-bath house totals about 20 DFU, which a 3-inch building drain carries at 1/4-inch-per-foot slope. The 3-inch minimum is usually set by the toilets, not the DFU count — a 2-inch pipe carries 21 DFU but is never allowed to serve a water closet.
Why does a water closet force a 3-inch drain?
IPC requires a 3-inch minimum drain for any water closet regardless of fixture-unit math, because a toilet discharges a large slug of water and solids that a 2-inch line cannot clear reliably. A 3-inch horizontal branch is also limited to two water closets — put three or more on a shared branch and you need 4-inch.
How does slope change drain capacity?
Steeper slope moves waste faster, so the pipe carries more fixture units. A 3-inch building drain carries 36 DFU at 1/8-inch per foot, 42 at 1/4-inch, and 50 at 1/2-inch (IPC Table 710.1(2)). Pipes 2.5-inch and smaller are not allowed at 1/8-inch per foot. The common residential default is 1/4-inch per foot.
Does this use IPC or UPC?
This tool uses IPC 2021 private-use fixture units. The Uniform Plumbing Code (UPC), adopted in a number of western states, assigns different fixture-unit values and drain capacities, so totals and sizes can differ. Confirm which code your jurisdiction has adopted before you rely on a number.

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Sources & references

Estimates for planning purposes only. Verify all results against the code edition adopted in your jurisdiction and with your Authority Having Jurisdiction (AHJ). This tool is not a substitute for a licensed plumber. See our methodology, sources, and code editions.