What is a Drainage Fixture Unit?

A DFU is a dimensionless value representing the probable discharge load a fixture places on the drainage system, based on its drain rate and frequency of use. IPC Table 709.1 assigns a DFU to each fixture so a system's total load can be summed.

How does DFU differ from WSFU?

WSFU measures supply-side water demand, while DFU measures drainage discharge load. A fixture can have different supply and drainage fixture-unit values because filling and draining behave differently, so the two are tabulated separately in the IPC.

Why is the stack size different from the branch size?

A vertical stack carries the combined load of every branch interval connected to it, so it usually serves a higher DFU total than any single horizontal branch and is sized from a separate stack column in IPC Table 710.1.

Does drain slope affect the size?

Yes. Horizontal drain capacity in IPC Table 710.1 is given by slope. This tool uses the common 1/8 inch per foot column; steeper slopes can carry slightly more load, but verify against the correct column for your installed slope.

Do I add vent fixture units too?

Vents are sized separately based on the DFU they serve and developed length, per the venting tables. This tool focuses on drain and stack sizing from the DFU total; size the vents in a separate step using the same fixture-unit load.

What is the Drainage Fixture Unit (DFU) Calculator?

Assigns Drainage Fixture Unit values from IPC Table 709.1 to each fixture, sums them per branch and stack, and looks up the minimum horizontal drain and vertical stack size from IPC Table 710.1. For plumbers sizing drain, waste, and vent systems. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Drainage Fixture Unit (DFU) Calculator

Name: Drainage Fixture Unit (DFU) Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

Get one useful tool a week

Like this tool? Enter your email and we'll send you one genuinely useful Gera tool a week — plus a link to come back to this one. No spam, one-click unsubscribe any time.

Sizing drain, waste, and vent piping starts by adding up the drainage load every fixture places on the system. This tool assigns the IPC Table 709.1 Drainage Fixture Unit value to each fixture, totals them, and looks up the minimum drain and stack size from IPC Table 710.1.

How it works

The load total drives both the branch and the stack size:

total DFU      = sum of (fixture count × fixture DFU)   (IPC Table 709.1)
horizontal drain = smallest size where DFU ≤ table max at 1/8"/ft slope
vertical stack   = smallest size where DFU ≤ stack table max

Branch and stack use different columns of IPC Table 710.1 because a stack collects the load of all the branch intervals feeding it, while a horizontal branch carries only the fixtures on that run.

Common fixture DFU values (IPC Table 709.1)

Fixture	DFU
Water closet (flush tank)	3
Bathtub or shower	2
Lavatory (bathroom sink)	1
Kitchen sink	2
Dishwasher (domestic)	2
Clothes washer (residential)	3
Floor drain (2-inch)	2
Floor drain (3-inch)	3
Urinal	4
Drinking fountain	0.5

These figures come from IPC Table 709.1. Local amendments exist; always verify against the edition and amendments adopted in your jurisdiction.

Worked example: two-bathroom house

Consider a two-bathroom residence with one kitchen and a laundry:

Bath 1: water closet (3) + lavatory (1) + tub (2) = 6 DFU
Bath 2: water closet (3) + lavatory (1) + shower (2) = 6 DFU
Kitchen: sink (2) + dishwasher (2) = 4 DFU
Laundry: clothes washer (3) = 3 DFU
Total: 19 DFU

At 19 DFU the horizontal building drain at 1/8-inch-per-foot slope requires a minimum 3-inch pipe (IPC Table 710.1 shows a 3-inch pipe handles up to 20 DFU on a horizontal drain). The vertical soil stack also sizes to 3 inches for the combined load. Any individual branch carrying a water closet must be at least 3 inches regardless of DFU, because the IPC specifies this minimum for water closet connections.

The water-closet minimum: a common oversight

The IPC imposes a hard minimum: any horizontal branch receiving a water closet must be at least 3 inches in diameter. A lavatory and sink on a 2-inch branch is fine, but the moment you add a toilet to that branch, the entire branch must increase to 3 inches. Many plan-check corrections trace back to this rule being overlooked when fixtures were added to an existing branch.

Stack versus branch: why they differ

A horizontal branch serves only the fixtures on its own run, so it is sized for its own DFU subtotal. The vertical stack accumulates every branch interval connected above, so it is sized for the cumulative DFU of all the branches it serves. In practice this means the stack is almost always larger than any individual branch, and the sizing comes from a different column of the table.

Slope and velocity

A 1/8-inch-per-foot slope (approximately 1%) is the minimum for horizontal drains and the column this tool uses. Steeper slopes increase velocity and self-cleaning action but also reduce the effective cross-section, so maximum DFU capacities in the IPC tables are keyed to the installed slope. Verify you are reading the column that matches your actual installed slope when comparing to the table.