TRAP SIZE
TRAP SIZE
IPC Table 1002.1
RESULT
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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.
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.
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.