What size drain pipe do I need for the number of fixtures I have?

Q: What is a drainage fixture unit (DFU)?

A DFU is a code-defined unit of probable drainage load assigned to each fixture type, used so that diverse fixtures can be summed onto one number for pipe sizing. The IPC and UPC publish DFU values and minimum trap sizes per fixture, and this tool uses whichever code you select. A lavatory is roughly 1 DFU while a water closet is several, reflecting their differing peak discharge.

Q: Does this size the vents too?

No. Vent sizing is a separate determination governed by IPC Section 906 or UPC Section 906, and you must verify it independently. The wizard also flags that trap-arm lengths beyond about 12 feet typically require a relief vent regardless of drain pipe size.

Question

What size drain pipe do I need for the number of fixtures I have?

Accepted Answer

The wizard converts each fixture to drainage fixture units (DFUs), sums them, and selects the smallest pipe whose table capacity meets or exceeds the total load. Capacities come directly from the IPC or UPC horizontal-branch and building-drain sizing tables, with the result citing the exact table used. It also enforces that the pipe is at least as large as the biggest trap on the branch.

What size drain pipe do I need for the number of fixtures I have?

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