9.1 Drainage Fixture Units and Load Estimation
Key Takeaways
- IPC Table 709.1 assigns 3 dfu to a private 1.6 gpf water closet, 4 dfu to a public 1.6 gpf water closet and 6 dfu to a public water closet flushing greater than 1.6 gpf.
- IPC Table 709.1 assigns 5 dfu to a bathroom group with a 1.6 gpf water closet and 6 dfu to a bathroom group with a water closet flushing greater than 1.6 gpf.
- IPC Table 709.2 assigns dfu values by trap size: 1 dfu for 1-1/4 inch, 2 for 1-1/2 inch, 3 for 2 inch, 4 for 2-1/2 inch, 5 for 3 inch and 6 for 4 inch.
- IPC 709.3 converts known flow to load on the basis that 1 gpm of flow is equivalent to two drainage fixture units.
- IPC Table 709.1 footnote b states that a showerhead over a bathtub or whirlpool attachment does not increase the drainage fixture unit value.
What a drainage fixture unit represents
IPC 709.1: drainage fixture unit values in Table 709.1 "designate the relative load weight of different kinds of fixtures" used in estimating the total load carried by a soil or waste pipe, for use with the sizing tables 710.1(1) and 710.1(2).
A dfu is a probability-weighted index, not a flow rate. It captures how much a fixture discharges, how fast, and how likely it is to discharge at the same moment as its neighbors. Do not confuse it with the water supply fixture unit (wsfu) from Appendix E — different system, different numbers for the same fixture. A private lavatory is 1 dfu on the drainage side and 0.7 wsfu on the supply side.
Table 709.1 — the values to know cold
| Fixture | dfu | Minimum trap |
|---|---|---|
| Bathroom group (1.6 gpf water closet) | 5 | — |
| Bathroom group (water closet flushing greater than 1.6 gpf) | 6 | — |
| Water closet, private (1.6 gpf) | 3 | note d |
| Water closet, private (greater than 1.6 gpf) | 4 | note d |
| Water closet, public (1.6 gpf) | 4 | note d |
| Water closet, public (greater than 1.6 gpf) | 6 | note d |
| Water closet, flushometer tank, public or private | 4 | note d |
| Urinal | 4 | note d |
| Urinal, 1 gallon per flush or less | 2 | note d |
| Urinal, nonwater supplied | 1/2 | note d |
| Lavatory | 1 | 1-1/4 in |
| Bidet | 1 | 1-1/4 in |
| Dental lavatory, dental unit or cuspidor | 1 | 1-1/4 in |
| Drinking fountain | 1/2 | 1-1/4 in |
| Bathtub (with or without overhead shower or whirlpool) | 2 | 1-1/2 in |
| Shower, flow 5.7 gpm or less | 2 | 1-1/2 in |
| Shower, greater than 5.7 to 12.3 gpm | 3 | 2 in |
| Shower, greater than 12.3 to 25.8 gpm | 5 | 3 in |
| Shower, greater than 25.8 to 55.6 gpm | 6 | 4 in |
| Kitchen sink, domestic (with or without disposer or dishwasher) | 2 | 1-1/2 in |
| Dishwashing machine, domestic | 2 | 1-1/2 in |
| Laundry tray (1 or 2 compartments) | 2 | 1-1/2 in |
| Combination sink and tray | 2 | 1-1/2 in |
| Service sink, sink | 2 | 1-1/2 in |
| Wash sink (circular or multiple), each set of faucets | 2 | 1-1/2 in |
| Automatic clothes washer, residential | 2 | 2 in |
| Automatic clothes washer, commercial | 3 | 2 in |
| Floor drain | 2 | 2 in |
| Emergency floor drain | 0 | 2 in |
The footnotes that change answers
- b. A showerhead over a bathtub or whirlpool bathtub attachment does not increase the dfu value. A tub-shower is 2 dfu, not 4.
- d. Trap size shall be consistent with the fixture outlet size — this is why water closets and urinals show "note d" instead of a number.
- e. For computing loads on building drains and sewers, water closets and urinals shall not be rated at a lower dfu unless the lower values are confirmed by testing.
- f. For fixtures added to a bathroom group, add the dfu value of those additional fixtures to the bathroom group count.
- g. See Section 406.2 for sizing the fixture drain, branch drain and drainage stack for an automatic clothes washer standpipe.
- h. Floor drains and floor sinks — see 709.4 and 709.4.1 for receptor values.
The bathroom group
A bathroom group is defined in IPC 202 as a group of fixtures consisting of a water closet, lavatory, and bathtub or shower, including or excluding a bidet, an emergency floor drain or both. Such fixtures are located together on the same floor level.
At 1.6 gpf, the group is 5 dfu. The same three fixtures counted individually would total 3 + 1 + 2 = 6 dfu. The group value is lower because a single bathroom serves one person at a time.
Footnote f is the trap: add a second lavatory to that bathroom and the count is 5 + 1 = 6 dfu, not a second group.
Fixtures not listed — Table 709.2
IPC 709.2: fixtures not listed in Table 709.1 take a dfu load based on the outlet size of the fixture. The minimum trap size for unlisted fixtures is the size of the drainage outlet but not less than 1-1/4 inches.
| Fixture drain or trap size | dfu |
|---|---|
| 1-1/4 inch | 1 |
| 1-1/2 inch | 2 |
| 2 inch | 3 |
| 2-1/2 inch | 4 |
| 3 inch | 5 |
| 4 inch | 6 |
Footnote a of Table 709.1 adds that for traps larger than 3 inches, use Table 709.2.
Note a useful sanity check: this table is a different scale from Table 709.1. A 2-inch floor drain is 2 dfu by Table 709.1 but a 2-inch unlisted fixture is 3 dfu by Table 709.2. Use Table 709.1 first; fall back to 709.2 only when the fixture is genuinely unlisted.
Converting flow to load — IPC 709.3
Where discharges to a waste receptor or to a drainage system are only known in gallons per minute, the drainage fixture unit values for those flows shall be computed on the basis that 1 gpm of flow is equivalent to two drainage fixture units.
A 15 gpm condensate and blowdown discharge from a mechanical unit is therefore 30 dfu. That single conversion often dominates a commercial drainage load calculation.
Indirect waste receptors — IPC 709.4
The drainage fixture unit load of an indirect waste receptor receiving the discharge of indirectly connected fixtures shall be the sum of the dfu values of the fixtures that discharge to the receptor, but not less than the dfu value given for the indirect waste receptor in Table 709.1 or 709.2.
709.4.1 Clear-water waste receptors: where floor drains, floor sinks and hub drains receive only clear-water waste from display cases, refrigerated display cases, ice bins, coolers and freezers, the receptor has a dfu value of one-half.
So a floor sink receiving three refrigerated case drains and nothing else is 1/2 dfu, not 2 dfu. The same floor sink also receiving a three-compartment sink takes the sum of the connected fixtures instead.
Worked load calculation: a small restaurant
| Fixture | dfu each | Count | Subtotal |
|---|---|---|---|
| Water closet, public, 1.6 gpf | 4 | 3 | 12 |
| Urinal, 1 gpf or less | 2 | 1 | 2 |
| Lavatory | 1 | 3 | 3 |
| Service sink | 2 | 1 | 2 |
| Three-compartment sink (2-inch trap, unlisted; use Table 709.2) | 3 | 1 | 3 |
| Domestic kitchen sink | 2 | 1 | 2 |
| Commercial dishwashing machine discharging 8 gpm (709.3: 1 gpm = 2 dfu) | 16 | 1 | 16 |
| Floor drains | 2 | 4 | 8 |
| Floor sink receiving only refrigerated case drains (709.4.1) | 0.5 | 2 | 1 |
| Automatic clothes washer, commercial | 3 | 1 | 3 |
| Total | 52 dfu |
That 52-dfu figure is what you carry into Table 710.1(1) to size the building drain, and it shows where the load really lives: the 8 gpm dishwasher alone accounts for 16 dfu, more than all the lavatories, sinks and the urinal combined.
Provisions for future fixtures
IPC 710.2: where provision is made for the future installation of fixtures, those fixtures shall be considered in determining the required sizes of drain pipes. Roughing in for a future basement bathroom means sizing the building drain for it today.
IPC 704.4: drainage piping for future fixtures shall terminate with an approved cap or plug.
What is the drainage fixture unit value of a bathroom group with a 1.6 gpf water closet under IPC Table 709.1?
A piece of equipment discharges 8 gpm to the drainage system. What drainage fixture unit load does IPC 709.3 assign?
A floor sink receives only clear-water waste from refrigerated display cases. What drainage fixture unit value applies?
A fixture is not listed in Table 709.1 and has a 2-inch trap. What dfu value and minimum trap size apply?