Drainage Fixture Units for a House and the Bathroom Group

Why this matters

Every drain and vent sizing table in both model codes is entered with drainage fixture units, so the count comes before the pipe. A basement bath added to an existing 3 in line, a second-floor addition tied into an old stack, a laundry moved across the house: each one is a DFU question first, and the plumber who skips the count is guessing at a pipe size. This card gives the residential values that agree between the codes, flags the ones that do not, explains the bathroom group as a single entry, and lays out a whole-house tally. Turning the tally into a pipe size is a separate method, owned by 'How to Size a Branch From Fixture Units Without Guessing'.

What a fixture unit is

A drainage fixture unit is a load rating, not a flow rate. It folds together how much a fixture discharges, how fast, and how often it is likely to discharge at the same time as the others on the pipe. The unit traces back to an early measurement of a lavatory's discharge, which is why the lavatory carries 1 and almost everything else is a multiple of it.

Two consequences follow, and both get misused:

• DFU add; gallons per minute do not. Ten fixtures at 2 DFU are 20 DFU, and the sizing table already accounts for the fact that they will not all drain at once. Converting each fixture to gpm and adding the gpm sizes the pipe for every fixture draining simultaneously, which never happens and oversizes the drain.
• A DFU value is only meaningful inside the code that published it. The IPC and UPC tables were built with different assumptions in places, so a value from one cannot be entered into the other's sizing table.

The residential values

Read from IPC 2021 Table 709.1 or UPC 2021 Table 702.1, as adopted by your jurisdiction. The rows below are the values where the two tables agree for ordinary residential fixtures.

Flagged: read these from your own table. The water closet and the clothes washer are where the codes and editions part company. The IPC lists water closets separately for private and public use and by flush volume (1.6 gpf and under, or more), and it lists a clothes washer on a standpipe as its own line. The UPC lists water closets by flush volume and type (gravity tank or flushometer) and a domestic clothes washer standpipe as its own line. The numbers in those rows are not the same between the two tables, and an older toilet over 1.6 gpf carries a higher value than a modern one in both. Look the row up for the toilet actually being installed, not the one assumed.

Shower flow. The IPC ties the shower's value to the total flow of its heads; a single standard head carries 2, and a multi-head or body-spray shower can carry more. A spa shower needs its own lookup.

Kitchen with disposer and dishwasher. Some editions list the combination on one line; others list the sink and dishwasher separately. Use whichever your table publishes and do not count the dishwasher twice.

The bathroom group as one entry

The IPC defines a bathroom group as a water closet, a lavatory, and a bathtub or shower, with or without a bidet or an emergency floor drain, located together on the same floor level. IPC Table 709.1 lists the group as a single entry, with a value depending on the toilet's flush volume.

Why it exists. The three fixtures in one bathroom are almost never draining together: one person is using the room. The group value is lower than the sum of its three fixtures counted individually, which is the code recognizing that a bathroom's fixtures share an occupant.

What qualifies. All three of water closet, lavatory, and tub or shower, in one room or suite on one floor. A powder room (toilet and lavatory only) is not a bathroom group; count its fixtures individually. A bath with two lavatories is commonly treated as a group plus the extra lavatory; check the definition and table notes in the adopted edition.

The two ways it goes wrong. Counting the group and then also adding the water closet, which doubles the toilet. And using the group value on a room that lacks one of the three fixtures, which undercounts it.

UPC. Check whether your adopted UPC edition's drainage table carries a bathroom group line or expects each fixture counted on its own. If it does not carry one, count each fixture individually.

Worked example: tallying a house

A two-story house getting a full bath added in the basement. Upstairs: two full baths (toilet, lavatory, tub-shower each). Main floor: a powder room, a kitchen sink with disposer and dishwasher, and a laundry with a standpipe washer. The question the tally answers is what the existing building drain will carry once the basement bath is on it.

The water closet and clothes washer values below are illustrative, entered as 3 each to show the method. Replace them with the rows your adopted table gives for the toilets and washer on this job before using the total.

 TALLY - individual fixtures, no group entry
 --------------------------------------------
 Bath 1   WC 3 + lav 1 + tub 2         =  6
 Bath 2   WC 3 + lav 1 + tub 2         =  6
 Powder   WC 3 + lav 1                 =  4
 Kitchen  sink 2 + dishwasher 2        =  4
 Laundry  washer 3                     =  3
                                  subtotal 23
 Basement WC 3 + lav 1 + shower 2      =  6
                              house total 29

Re-add from the other direction. Toilets: 4 at 3 = 12. Lavatories: 4 at 1 = 4. Tubs and shower: 3 at 2 = 6. Kitchen sink and dishwasher: 2 at 2 = 4. Washer: 3. That is 12 + 4 + 6 + 4 + 3 = 29, which matches the tally.

With a group entry. If the adopted table carries a bathroom group line, the three full baths (Bath 1, Bath 2 and the basement) each enter once at the group value instead of at 6. The powder room still enters at 4, because it has no tub or shower. Whatever the group value in your table, it replaces the three individual lines for each of the three full baths, and the total falls by three times the difference between 6 and the group value.

Load on each pipe, not just the house. The basement bath's own horizontal branch carries only its 6 (or the group value). The upstairs stack carries Baths 1 and 2, 12 by individual count. The building drain downstream of the last connection carries all 29. Each pipe is sized for the DFU upstream of it, so the tally is kept by location, not as one number.

What the tally is for. Take the load on each pipe, with its slope, to the sizing table in the same code the values came from, by the method on 'How to Size a Branch From Fixture Units Without Guessing'. Whether an existing 3 in building drain carries 29 depends on the slope and on any limit the table places on how many water closets a 3 in drain may serve, which is a sizing-table read, not something this tally settles.

What would change the answer. If two of the existing toilets are old 3.5 gpf units, their rows in either table carry a higher value than a 1.6 gpf toilet, and swapping them for 1.6 gpf toilets as part of the job lowers the load on the drain. That is a legitimate way to add a fixture to a borderline drain, and the inspector will want to see the new toilets on the permit.

A second branch of the same job: the basement is below the sewer. If the basement floor sits below the building drain, the basement bath drains by gravity into a sewage ejector basin, and its 6 DFU size the piping from the fixtures to the basin. What reaches the building drain is no longer 6 DFU of gravity flow; it is the pump's discharge. The codes convert a pump's discharge rate into a fixture-unit load on the gravity drain it empties into (IPC Section 712 in substance; check the current edition and the UPC's equivalent), so the tally line for the basement changes from a fixture count to a converted pump load, and that figure can be larger or smaller than 6. A tally that keeps the basement at 6 after an ejector is designed in is counting the wrong thing.

The same count feeds the vents. Vent sizing tables and the wet vent tables are entered with DFU as well, so the per-pipe subtotals above get reused when the vent for the basement bath is sized. Keep the tally sheet with the job; the venting card will ask for it.

Where the count goes wrong in the field

• Counting from the old permit. A house that has been remodeled since the original drawings carries fixtures nobody counted: a wet bar, a laundry sink, a basement shower roughed in and never finished. Walk the house and count what is there, including capped rough-ins that will be finished.
• Mixing tables. A DFU from the IPC entered into a UPC sizing table, or the reverse. The job file names one code; every value comes from it.
• Leaving out the floor drain. A basement floor drain carries a value in both tables; an emergency floor drain that receives no regular discharge commonly carries zero. Know which you have.

How to verify you got this right

• Every fixture in the house, including capped rough-ins, appears on the tally with its location.
• The water closet and clothes washer values on the tally match the rows in the adopted table for the actual fixtures.
• A bathroom group entry is used only for a room with a water closet, a lavatory and a tub or shower, and the water closet is not counted again.
• The total re-adds by fixture type to the same number.
• Each pipe on the layout carries its own upstream subtotal.

References

• IPC 2021 Table 709.1 and the bathroom group definition in Chapter 2, as adopted by your jurisdiction.
• UPC 2021 Table 702.1, as adopted by your jurisdiction.
• See related: How to Size a Branch From Fixture Units Without Guessing.
• See related: Wet Venting a Bathroom Group Under IPC and UPC; Where the IPC and UPC Disagree on Service and Remodel Work.