Trap Size Chart — Minimum Trap & Trap Arm Sizes
The minimum trap size for every listed plumbing fixture, arranged by trap size so you can read a 1-1/4", 1-1/2", 2", or 3" trap's fixture list at a glance. IPC and UPC values are kept separate — the UPC's printed size governs both the trap and its trap arm, and the two codes genuinely disagree on fixtures like showers. IPC values per Table 709.1 (2021 edition; unchanged 2015–2024); UPC values per Table 702.1 (2021 edition).
IPC — minimum trap size by fixture
| Fixture | Min trap | DFU |
|---|---|---|
| Lavatory | 1-1/4" | 1 |
| Dental lavatory | 1-1/4" | 1 |
| Dental unit or cuspidor | 1-1/4" | 1 |
| Bidet | 1-1/4" | 1 |
| Drinking fountain | 1-1/4" | 0.5 |
| Kitchen sink, domestic | 1-1/2" | 2 |
| Kitchen sink, domestic (with food-waste disposer and/or dishwasher) | 1-1/2" | 2 |
| Sink | 1-1/2" | 2 |
| Service sink | 1-1/2" | 2 |
| Wash sink, circular or multiple (per set of faucets) | 1-1/2" | 2 |
| Combination sink and tray | 1-1/2" | 2 |
| Laundry tray (1 or 2 compartments) | 1-1/2" | 2 |
| Bathtub (with or without overhead shower or whirlpool) | 1-1/2" | 2 |
| Shower, total flow 5.7 gpm or less | 1-1/2" | 2 |
| Dishwashing machine, domestic | 1-1/2" | 2 |
| Shower, over 5.7 to 12.3 gpm | 2" | 3 |
| Automatic clothes washer, residential | 2" | 2 |
| Automatic clothes washer, commercial | 2" | 3 |
| Floor drain | 2" | 2 |
| Emergency floor drain | 2" | 0 |
| Floor sink | 2" | — |
| Shower, over 12.3 to 25.8 gpm | 3" | 5 |
| Shower, over 25.8 to 55.6 gpm | 4" | 6 |
IPC — fixtures with integral traps
| Fixture | Trap size | DFU |
|---|---|---|
| Water closet, private (1.6 gpf) | Matches outlet | 3 |
| Water closet, private (over 1.6 gpf) | Matches outlet | 4 |
| Water closet, public (1.6 gpf) | Matches outlet | 4 |
| Water closet, public (over 1.6 gpf) | Matches outlet | 6 |
| Water closet, flushometer tank (public or private) | Matches outlet | 4 |
| Urinal | Matches outlet | 4 |
| Urinal (1.0 gpf or less) | Matches outlet | 2 |
| Urinal, nonwater-supplied | Matches outlet | 0.5 |
Unlisted fixtures — DFU by trap size
| Drain or trap size | DFU |
|---|---|
| 1-1/4" | 1 |
| 1-1/2" | 2 |
| 2" | 3 |
| 2-1/2" | 4 |
| 3" | 5 |
| 4" | 6 |
UPC — minimum trap and trap arm size by fixture
| Fixture | Min trap & arm |
|---|---|
| Lavatory | 1-1/4" |
| Dental unit or cuspidor | 1-1/4" |
| Drinking fountain or water cooler | 1-1/4" |
| Bidet | 1-1/4" |
| Urinal, exposed trap | 1-1/2" |
| Lavatories, in sets | 1-1/2" |
| Washfountain | 1-1/2" |
| Bidet | 1-1/2" |
| Kitchen sink, domestic (with or without food-waste disposer and/or dishwasher) | 1-1/2" |
| Laundry sink (with or without clothes-washer discharge) | 1-1/2" |
| Bar sink | 1-1/2" |
| Bar sink | 1-1/2" |
| Commercial sink, with food waste | 1-1/2" |
| Exam-room sink | 1-1/2" |
| Special-purpose sink | 1-1/2" |
| Bathtub or combination bath/shower | 1-1/2" |
| Dishwasher, domestic (independent drain) | 1-1/2" |
| Indirect waste receptor (low demand) | 1-1/2" |
| Urinal, integral trap (1.0 gpf or less) | 2" |
| Urinal, integral trap (over 1.0 gpf) | 2" |
| Urinal, nonwater (with drain-cleansing action) | 2" |
| Urinal, hybrid | 2" |
| Washfountain | 2" |
| Special-purpose sink | 2" |
| Service sink or mop basin | 2" |
| Shower, single-head trap | 2" |
| Shower, multi-head (each additional head) | 2" |
| Clothes washer, domestic (standpipe) | 2" |
| Food-waste disposer, commercial | 2" |
| Floor drain | 2" |
| Floor drain, emergency | 2" |
| Indirect waste receptor (moderate or heavy demand) | 2" |
| Water closet, 1.6 gpf gravity tank | 3" |
| Water closet, 1.6 gpf flushometer tank | 3" |
| Water closet, 1.6 gpf flushometer valve | 3" |
| Water closet, over 1.6 gpf gravity tank | 3" |
| Water closet, over 1.6 gpf flushometer valve | 3" |
| Clinical sink | 3" |
| Special-purpose sink | 3" |
| Service sink or mop basin | 3" |
| Service sink, flushing rim | 3" |
| Mobile home, trap | 3" |
| Indirect waste receptor | 3" |
Trap vs. trap arm — why the UPC prints one number for both
The trap arm is the pipe between the trap weir and its vent. The UPC sizes the two together: the printed minimum in Table 702.1 applies to the trap and the trap arm as a unit, which is why UPC trap arms can never neck down below the trap size. The IPC prints only the trap minimum in Table 709.1; the fixture drain downstream then sizes by its drainage fixture units. Either way the trap can never be larger than the fixture drain it discharges to — a trap that big won't scour.
For the drainage loads these traps carry, see the DFU chart, and size the downstream piping with the drain pipe size chart.
The codes disagree — check your local amendments
The shower row is the clearest fork: 1-1/2" under the IPC for a standard-flow shower, 2" always under the UPC. Kitchen sinks, lavatories, and laundry fixtures happen to agree, but never assume a value crosses code families. Plumbing is heavily amended locally — a state or city can rewrite any row of either table — so confirm the enforced code, edition, and amendments with your jurisdiction before rough-in.
Common questions
What size trap does a kitchen sink need?
A domestic kitchen sink takes a 1-1/2" trap under both codes — IPC Table 709.1 and UPC Table 702.1 agree, with or without a food-waste disposer or dishwasher discharging through it. Under the UPC the 1-1/2" minimum also applies to the trap arm.
What size trap does a shower need?
Here the codes fork. The UPC requires a 2" trap and trap arm for every shower. The IPC scales with flow: 1-1/2" up to 5.7 GPM of total spray, 2" over 5.7 to 12.3 GPM, 3" up to 25.8 GPM, and 4" beyond that. A standard single-head shower is 1-1/2" IPC territory but still 2" under the UPC.
Why is there no trap size listed for a water closet?
Water closets and most urinals have the trap built into the fixture itself. The IPC prints no minimum and instead requires the connection to match the fixture outlet (its note d); the UPC prints 3" for water closets because that is the minimum trap-arm and drain connection the fixture needs, not a site-built trap.
Can I install a larger trap than the minimum?
Minimums are floors, but both codes warn against oversizing: a trap so large the fixture discharge cannot scour it will collect solids and lose its seal. The UPC states this directly in Table 702.1’s footnotes. Size the trap to the fixture, not to the pipe you happen to have.
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