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DWV pipe size calculator

To size a drain, add up the drainage fixture units (DFU) of every fixture it serves, then take the smallest pipe whose IPC rating covers that load. Building drains are also rated by slope, and a building drain that serves a water closet is never smaller than 3 inches. Enter your fixtures and the calculator does both steps.

Code basis: International Plumbing Code (IPC) Tables 709.1, 710.1(1) and 710.1(2), values as published in the 2015 to 2024 editions, as transcribed in DWV Pro. Not affiliated with or endorsed by the ICC. International Plumbing Code and IPC are trademarks of the International Code Council, Inc. Last checked 14 October 2026.

Enter how many of each fixture the run serves.

Fixture groups
5 DFU each
6 DFU each
Water closets and urinals
3 DFU each
4 DFU each
4 DFU each
6 DFU each
4 DFU each
2 DFU each
0.5 DFU each
Bathroom
1 DFU each
2 DFU each
2 DFU each
3 DFU each
1 DFU each
Kitchen and laundry
2 DFU each
2 DFU each
2 DFU each
2 DFU each
3 DFU each
2 DFU each
2 DFU each
Other
2 DFU each
2 DFU each
2 DFU each
0.5 DFU each
Run

Replaces the fixture total, as in the app.

Result

Add fixtures to see the minimum pipe size.

DWV Pro runs this offline on iPhone and saves the job.

Worked example

3" building drain

Computed by the same code as the calculator above.

A duplex remodel ties two bathrooms and a kitchen into one building drain, run at 1/4 inch per foot.

FixtureCountDFU eachDFU
Water closet, private (1.6 gpf)236
Lavatory212
Bathtub or whirlpool122
Kitchen sink, domestic122
Total load12
  1. Total the load: 12 DFU (IPC Table 709.1 values).
  2. Find the smallest building drain rated for 12 DFU at 1/4 inch per foot. On load alone that is 2", rated for 21 DFU (IPC Table 710.1(1)).
  3. The drain serves water closets, so it is held to the 3-inch minimum. The answer is 3", rated for 42 DFU at that slope.

Limits, and when to ask your inspector

  • A planning aid, not a design or approval. The installer and the inspector are responsible for code compliance.
  • These are International Plumbing Code values. This is not a Uniform Plumbing Code (UPC) calculator: the UPC assigns its own fixture unit values and sizing tables, so results there can differ.
  • It sizes a drain from its load. It does not size vents, trap arms or cleanouts, and the 3-inch water closet minimum is applied to building drains only.
  • A load beyond the largest size in the table returns no answer. That job needs an engineer.
  • Ask your inspector before you install if your jurisdiction has adopted a different code edition or local amendments, if a fixture isn’t on the list (enter the code’s value with the DFU override), if you are tying into an existing drain, or if the load sits right at a size’s rating.

Save jobs and get the full tool in DWV Pro

Vent sizing, slope and fall, invert elevations, saved projects and a PDF sizing summary, offline on your iPhone. Free download with a one-time unlock.

Download DWV Pro on the App StoreDWV Pro details

Questions

What is a drainage fixture unit?

A drainage fixture unit (DFU) is the plumbing code’s measure of the load a fixture puts on the drain, based on how much it discharges and how often it is used. In IPC Table 709.1 a lavatory is 1 DFU, a private 1.6 gpf water closet is 3, and a bathroom group with a 1.6 gpf water closet is 5.

Why does it say 3 inches when the load fits 2-inch pipe?

Under the IPC, a building drain that serves a water closet must be at least 3 inches. The calculator applies that minimum whenever your fixtures include a water closet or a bathroom group and the run type is a building drain or sewer.

Does slope change the drain size?

For building drains and sewers, yes. IPC Table 710.1(1) rates each size by slope, so a steeper drain carries more. 26 DFU fits a 2-inch drain at 1/2 inch per foot but needs 3 inches at 1/4 inch per foot. The table only lists 1/16 inch per foot for 8-inch pipe and larger.

Does it work for the Uniform Plumbing Code?

No. The values are from the International Plumbing Code. The UPC uses its own fixture unit values and tables, so check which code your jurisdiction has adopted.

What does the DWV Pro app add?

The app does this tally and drain sizing free and offline. Vent & Slope, a one-time $4.99 purchase, adds vent sizing from developed length, slope and fall, invert elevations, saved projects and a PDF sizing summary.

More free tools

All free web tools or see tools for the trades.