DWV sizing — DFU load, branch/vent diameter, and the trap-arm distance limit
WC (3 DFU) + lavatory (1 DFU) + bathtub (2 DFU) = 6 DFU → 3″ branch (floored by the WC), 1.5″ vent. The bathtub's 1.5″ trap must reach its vent within 6 ft, sloped ¼″/ft (IPC Table 909.1).
What this diagram shows
A horizontal drain branch sloping toward a soil stack, with a water closet, lavatory, and bathtub draining into it and a running fixture-unit total climbing 3, 4, 6 as each is added. The branch is labeled a 3-inch pipe, floored at that size because a water closet is present. A vent riser rises from near the bathtub connection, above a dashed flood-level-rim line, labeled a 1.5-inch vent, half the branch diameter. A red dimension marks the horizontal trap-arm distance from the bathtub's trap to the vent connection, capped at 6 feet for its 1.5-inch trap sloped at 1/4 inch per foot, while a note explains the water closet's own trap arm has no length limit.
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DWV Pipe Size Calculator
Free DWV pipe size calculator — enter your fixtures and get branch, building drain, and vent sizes from IPC Table 709.1 & 710.1, plus a trap-arm distance check.
