TRAP SIZE · REVIEWED MAY 2026 · BY BRENT

TRAP SIZE

IPC Table 1002.1

Fixture type

RESULT

—

FILL IN ABOVE

IPC Table 1002.1. WCs have integral 3" traps cast into the porcelain — no separate trap needed. Estimate only — verify with a licensed plumber and local plumbing code/inspector before purchase or installation. Not a substitute for engineered drawings.

RECOMMENDED TOOLS

Plumbing Tools we recommend for projects like this

→

About this calculator

Every plumbing fixture needs a trap — a U-shape that holds water as a barrier against sewer gas — and the IPC specifies a minimum trap size for each fixture type in Table 1002.1. Use this trap size calculator to confirm you're installing the right trap diameter when replacing a fixture, roughing in new construction, or troubleshooting a slow-draining or gurgling fixture (often a sign the trap is undersized or the trap arm too long). The output also includes DFU value and minimum vent size, so you can size the rest of the branch in one pass. ESTIMATE ONLY — verify with a licensed plumber and local plumbing code before installation.

How to use this calculator

Pick the fixture type from the dropdown. The calculator returns the minimum trap diameter, the minimum branch drain size, the DFU value (used for drain and vent sizing), and the minimum vent size for the trap.

Most residential fixtures use 1¼" traps (lavs, bidets) or 1½" traps (kitchen sinks, tubs, washers, dishwashers). Showers and floor drains need 2". Toilets have integral 3" traps cast into the porcelain — no separate trap or P-trap install needed.

The trap arm (horizontal pipe between trap and vent) is limited to 5× the trap diameter per IPC 1002.4. Exceeding this length causes traps to siphon dry, letting sewer gas into the home.

Worked example

For a kitchen sink installation:

Trap size: 1½". Min branch drain: 1½". DFU: 2. Min vent: 1¼". Trap arm max: 5 × 1.5 = 7.5 ft from trap to vent.

For a shower stall: Trap size: 2" (larger than lavs/sinks because of higher peak flow). Branch drain: 2". DFU: 2. Vent: 1½". Trap arm max: 10 ft.

For a toilet: No separate trap — it's built into the bowl. Branch drain: 3" minimum. DFU: 3. Vent: 2".

For a clothes washer standpipe: Trap: 2". Branch: 2". DFU: 3. Vent: 1½". Standpipe must be 18-30" tall above the trap weir per IPC 802.4.

For a basement floor drain: Trap: 2". Branch: 2". DFU: 2. Vent: 1½". Floor drains in unheated spaces or unused locations need a trap primer to prevent the trap from drying out.

Common mistakes & waste factors

Using a 1¼" trap on a kitchen sink. Code minimum is 1½" — the higher discharge volume of food disposal and dishwashing exceeds 1¼" trap capacity. Slow-draining sinks are often a sign of an undersized trap.

Forgetting the trap arm length limit. Trap arm > 5× trap diameter = siphoning trap = sewer gas in the house. Add a vent closer to the trap if the run is too long.

Using flexible accordion P-traps. Code prohibits flexible accordion traps because their corrugations trap debris. Always use rigid PVC or chrome-plated brass traps.

Forgetting the trap primer on rarely-used drains. Floor drains and infrequently-used fixtures dry out their traps within 2-3 weeks. Install a trap primer (auto-fills the trap on each use of an adjacent fixture) or accept periodic mopping.

Rules of thumb

IPC Table 1002.1 trap sizes: 1¼" for lav/bidet/drinking fountain, 1½" for kitchen sink/bar sink/tub/dishwasher/laundry tub, 2" for shower/floor drain/washer/urinal, integral 3" for toilet.

Trap arm max length: 5× trap diameter per IPC 1002.4. (1¼" trap → 6.25" max arm; 2" trap → 10" max arm.)

Vent must be ≥ ½ trap size, never less than 1¼".

DFU: lav 1, sink 2, tub 2, shower 2, washer 3, WC 3, floor drain 2.

Trap primers: required on any drain that won't see weekly use. Without one, the trap dries and sewer gas enters the home.

Never install reverse-pitch traps (S-traps) — banned by IPC because they self-siphon dry every drain cycle.

Common questions

Why does my fixture have a specific trap size?

Trap size has to match the flow capacity of the fixture so it self-scours — water leaving the fixture has enough velocity to flush the trap clean of debris. Too-small traps clog. Too-large traps don't maintain flow velocity and let solids settle in the bottom of the U. IPC Table 1002.1 picks the size that self-scours for each fixture's typical discharge.

How long can a trap arm be?

IPC 1002.4 limits the trap arm (the horizontal pipe between the trap weir and the vent) to a length that depends on trap size: 1¼" trap → 5 ft max arm, 1½" → 6 ft, 2" → 8 ft, 3" → 12 ft. Going longer breaks the trap seal under flow. The arm must also slope ¼" per foot back toward the drain so it self-drains.

Do toilets need a separate trap?

No — every modern water closet has an integral 3-inch S-trap or P-trap molded into the porcelain. That's the curved water column you see in the bowl. The branch drain leaving the toilet is straight pipe to the stack. This is why toilets need a 3" minimum branch even if other fixtures sharing the line would allow smaller.