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

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 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.

Drain Sizing Wizard — Step-by-Step DWV Sizing

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.

Run the numbers for your project

Use the Drain Sizing Wizard — Step-by-Step DWV Sizing to get an exact answer for your inputs.

Open the calculator

Does this size the vents too?

Run the numbers for your project

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