Drainage Fixture Unit Calculator — DFU Totals
Every drain, branch, stack and sewer is sized from one number: the total drainage fixture units discharging into it. Count your fixtures and this gives you that total from IPC Table 709.1, the minimum trap size for each one, and the drain size the load requires — including the water-closet rule that overrides the table.
Count your fixtures
Building drains and sewers only.
Residential fixtures
Commercial fixtures
Total drainage fixture units
The table alone allows 2", but a water closet on a building drain floors it at 3 inches.
Minimum drain size
3"
That size carries
42 DFU
The water-closet floor applied
IPC Table 710.1(1)'s footnote sets a 3-inch minimum on any building drain serving a water closet, regardless of DFU load. The load alone would have allowed a smaller pipe.
See the breakdown
Planning estimate based on IPC 2021 Tables 709.1, 709.2 and 710.1. UPC jurisdictions assign some fixtures different values. A licensed plumber and your AHJ have final say.
The method, explained in plain English
Addition, a table lookup, and one footnote that overrides both.
A fixture unit is not gallons
It is a dimensionless weight bundling discharge rate, duration and frequency of use. That is what lets one number compare a floor drain with a urinal.
Groups beat the sum of the parts
A bathroom group is 5 DFU where its fixtures counted individually give 6, because one person cannot use them all at once. Always use the group value when the fixtures share a bathroom.
A shower over a tub is free
Table 709.1 says outright that a showerhead over a bathtub, or a whirlpool attachment, does not increase the tub's value. It stays at 2 DFU because only one can drain at a time.
Trap size is a separate rule
The DFU value and the minimum trap size come from the same table but are independent requirements. A 2 DFU kitchen sink needs a 1-1/2 inch trap; a 2 DFU floor drain needs 2 inch.
Worked examples
A house, a single bathroom branch, and a small commercial washroom.
Two-bathroom house building drain
1 × kitchen sink (2) = 2 · 1 × dishwasher (2) = 2
1 × clothes washer (2) = 2 · 1 × laundry tray (2) = 2
total = 18 DFU
Table 710.1(1) at 1/4 in/ft: 2" carries 21 DFU → 2" would do
water-closet footnote → 3 inch minimum
Result: 3 inch, carrying 42 DFU — more than double the load. The footnote, not the arithmetic, decided it.
One bathroom on a horizontal branch
Table 710.1(2) horizontal branch: 1-1/2" = 3, 2" = 6
→ 2 inch on load alone
but a water closet needs a 3" branch in practice — Table 710.1(2)
footnotes cap a 3" branch at two water closets
Result: the DFU load says 2 inch, but you cannot run a water closet on a 2-inch branch. This is why the 3-inch closet bend is universal in residential work — and why load is only ever half the sizing question.
Small commercial washroom
Four public 1.6 gpf water closets, two urinals, four lavatories and a service sink on a building drain at 1/8 in/ft.
4 × lavatory (1) = 4 · 1 × service sink (2) = 2
total = 30 DFU
Table 710.1(1) at 1/8 in/ft: 3" carries 36 DFU → 3 inch ✓
at 1/4 in/ft the same 3" carries 42
Result: 3 inch even on the flatter grade, with 6 DFU spare. Note how much heavier commercial fixtures are — four public water closets alone nearly match the whole house in example 1.
IPC Table 709.1 — DFU values and trap sizes
Rendered from the same data the calculator uses, so the two cannot disagree. Fixtures with no trap size shown have an integral trap.
| Fixture | DFU | Min trap | Type |
|---|---|---|---|
| Bathroom group (1.6 gpf water closet) | 5 | integral | Residential |
| Bathroom group (over 1.6 gpf water closet) | 6 | integral | Residential |
| Water closet, private (1.6 gpf) | 3 | integral | Residential |
| Water closet, private (over 1.6 gpf) | 4 | integral | Residential |
| Lavatory | 1 | 1-1/4" | Residential |
| Bathtub (with or without shower) | 2 | 1-1/2" | Residential |
| Shower, up to 5.7 gpm | 2 | 1-1/2" | Residential |
| Shower, over 5.7 to 12.3 gpm | 3 | 2" | Residential |
| Kitchen sink, domestic | 2 | 1-1/2" | Residential |
| Dishwashing machine, domestic | 2 | 1-1/2" | Residential |
| Clothes washer, residential | 2 | 2" | Residential |
| Laundry tray (1 or 2 compartments) | 2 | 1-1/2" | Residential |
| Bidet | 1 | 1-1/4" | Residential |
| Floor drain | 2 | 2" | Residential |
| Sink | 2 | 1-1/2" | Residential |
| Water closet, public (1.6 gpf) | 4 | integral | Commercial |
| Water closet, public (over 1.6 gpf) | 6 | integral | Commercial |
| Water closet, flushometer tank | 4 | integral | Commercial |
| Urinal | 4 | integral | Commercial |
| Urinal, 1 gpf or less | 2 | integral | Commercial |
| Urinal, nonwater supplied | 0.5 | integral | Commercial |
| Shower, over 12.3 to 25.8 gpm | 5 | 3" | Commercial |
| Shower, over 25.8 to 55.6 gpm | 6 | 4" | Commercial |
| Service sink | 2 | 1-1/2" | Commercial |
| Clothes washer, commercial | 3 | 2" | Commercial |
| Drinking fountain | 0.5 | 1-1/4" | Commercial |
| Wash sink (per faucet set) | 2 | 1-1/2" | Commercial |
| Dental lavatory | 1 | 1-1/4" | Commercial |
IPC Table 709.2 — fixtures not otherwise listed
When a fixture is not in Table 709.1, its drain connection decides its value.
| Drain connection | DFU value |
|---|---|
| 1-1/4" | 1 |
| 1-1/2" | 2 |
| 2" | 3 |
| 3" | 5 |
| 4" | 6 |
Sources & standards: IPC 2021 Table 709.1 (drainage fixture unit values), Table 709.2 (fixtures not listed), Table 710.1(1) (building drains and sewers) and Table 710.1(2) (horizontal branches and stacks), including the footnote setting a 3-inch minimum on a building drain serving a water closet. Roughly fifteen states use the UPC or a derivative, which assigns some fixtures different values. Local amendments override the model code.
Frequently asked questions
Common questions about drainage fixture units, trap sizes, and the water-closet rule.
How many DFU is a bathroom group?
A bathroom group with a 1.6 gpf water closet is 5 DFU under IPC Table 709.1; with a water closet over 1.6 gpf it is 6. That is deliberately less than counting the fixtures separately (3 + 1 + 2 = 6), because the fixtures in one bathroom compete for one person and rarely discharge together.
How many DFU is a typical house?
Two bathroom groups plus a kitchen sink, dishwasher, clothes washer and laundry tray comes to 18 DFU — the defaults above. That is comfortably inside a 3-inch building drain, which carries 42 DFU at 1/4 inch per foot.
Why does 18 DFU still need a 3-inch drain?
Because of the water closet, not the load. Table 710.1(1) would carry 18 DFU on a 2-inch building drain at 1/4 in/ft, but the table's own footnote floors any building drain serving a water closet at 3 inches. This is the single most commonly missed line in residential drainage sizing, and the calculator applies it and tells you when it bit.
What is the difference between DFU and WSFU?
They measure opposite ends of the same fixture. DFU weights what a fixture puts down the drain; WSFU weights how hard it pulls on the supply. The same lavatory is 1 DFU and 0.7 WSFU. Use the Water Supply Fixture Unit Calculator for the pressure side.
What if my fixture isn't on the list?
IPC Table 709.2 handles it: a fixture not listed in Table 709.1 takes its DFU value from the size of its drain connection — 1 DFU for 1-1/4 inch, 2 for 1-1/2, 3 for 2 inch, 5 for 3 inch and 6 for 4 inch. Those rows are in the reference tables below.
Does a shower over a bathtub count twice?
No. A showerhead over a bathtub does not increase the tub's drainage fixture unit value — it is still 2 DFU, because only one of them can drain at a time. The same logic applies to a whirlpool attachment.
Why do some fixtures have no trap size?
Because they trap themselves. Water closets and urinals have an integral trap built into the china, so Table 709.1 gives them no separate minimum trap size — the fixture is the trap. Everything else needs a trap sized at or above the value shown.
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