Fixture Unit Calculator
Total up drainage fixture units (DFU) and water supply fixture units (WSFU) from your fixture counts — the two numbers every drain-size and supply-size table starts from. Build the fixture list, and the calculator sums both columns from verified code values (IPC Table 709.1 for DFU, IPC Appendix E for WSFU, private use), returns the minimum building-drain size for the DFU total, and prints a fixture unit schedule you can attach to a permit package. Free, no login.
Fixtures
Fixture Units
DFU per IPC Table 709.1 and WSFU per IPC Appendix E Table E103.3(2), private use (hose bibb & bar sink per UPC 610.3/702.1 — the IPC tables have no row for them). Drain size from IPC Table 710.1(1) at 1/4"/ft; a drain serving any water closet is 3" minimum.
Fixture Unit Schedule
Drainage (DFU) and water-supply (WSFU) fixture-unit tabulation — prepared with the NORDIX Fixture Unit Calculator (nordixhq.com/tools/fixture-unit-calculator)
| Fixture | Qty | DFU ea | DFU | WSFU ea | WSFU |
|---|---|---|---|---|---|
| Water closet (1.6 gpf, flush tank) | 1 | 3 | 3 | 2.2 | 2.2 |
| Lavatory | 1 | 1 | 1 | 0.7 | 0.7 |
| Totals | 2 | 4 | 2.9 |
| Minimum building drain | 2" at 1/4"/ft (IPC Table 710.1(1)); any drain serving a water closet is 3" minimum |
| Water supply demand basis | 2.9 WSFU total — carry into the IPC Appendix E water-supply sizing procedure |
Basis: DFU per IPC Table 709.1 (private use); WSFU per IPC Appendix E Table E103.3(2), private “Total” column. Hose bibb and bar sink values are UPC Tables 610.3 / 702.1 (private) — the IPC tables carry no row for them. Minimum building-drain size from IPC Table 710.1(1) at 1/4" per foot slope. This is a design aid; fixture classifications, occupancy (private vs public), the adopted code edition, and the AHJ govern the final sizing.
How this calculator works
Fixture units are the plumbing codes’ probability-weighted load ratings — a way to add up fixtures that are rarely all running at once. Each fixture carries two separate values, and the calculator sums both:
total DFU = Σ (count × DFU) · total WSFU = Σ (count × WSFU)
The per-fixture values are looked up from verified code tables, never re-typed: DFU from IPC Table 709.1 (private use) and WSFU from IPC Appendix E Table E103.3(2), private “Total” column. Two fixtures the IPC tables have no row for — the hose bibb (Appendix E treats it as continuous GPM demand, not fixture units) and the bar sink — carry verified UPC values (Tables 610.3 / 702.1) and are labeled as such in the picker. The DFU total then drives the minimum building-drain size through IPC Table 710.1(1) at the 1/4"-per-foot slope column, with one caveat the generic lookup cannot see: any drain serving a water closet is 3" minimum regardless of DFU (the table’s note a). The WSFU total is your entry point into the water-supply sizing procedure — it converts to design GPM through the tabulated Hunter’s curve, IPC Table E103.3(3).
Worked example
A two-bath house core: 2 water closets, 2 lavatories, 1 bathtub/shower, 1 kitchen sink, 1 clothes washer.
- DFU: 2×3 + 2×1 + 2 + 2 + 2 = 14.0 DFU.
- WSFU: 2×2.2 + 2×0.7 + 1.4 + 1.4 + 1.4 = 10.0 WSFU.
- Drain size: 14 DFU fits the 2" cap (21 DFU at 1/4"/ft) in IPC Table 710.1(1), so the calculator reports 2" — but because the load includes water closets, the governing minimum is 3" per note a. In practice a house drain like this is run at 3" or 4".
Build the same list above (the calculator starts with a water closet and lavatory) to reproduce it, then print the schedule.
Reference: common DFU values (IPC Table 709.1)
An excerpt of the verified table behind the calculator — drainage fixture units and minimum trap sizes for the fixtures that appear on nearly every job. Water-closet traps follow the fixture outlet rather than a fixed minimum.
| Fixture | DFU | Min trap |
|---|---|---|
| Bathroom group (1.6 gpf water closet) | 5 | — |
| Water closet, private (1.6 gpf) | 3 | — |
| Water closet, public (1.6 gpf) | 4 | — |
| Lavatory | 1 | 1-1/4" |
| Kitchen sink, domestic | 2 | 1-1/2" |
| Bathtub (with or without overhead shower or whirlpool) | 2 | 1-1/2" |
| Shower, total flow 5.7 gpm or less | 2 | 1-1/2" |
| Automatic clothes washer, residential | 2 | 2" |
| Dishwashing machine, domestic | 2 | 1-1/2" |
| Laundry tray (1 or 2 compartments) | 2 | 1-1/2" |
| Floor drain | 2 | 2" |
| Drinking fountain | 0.5 | 1-1/4" |
Frequently asked questions
What is a drainage fixture unit (DFU)?
A DFU is a probability-weighted load rating, not a flow rate. Fixtures rarely discharge at the same time, so instead of adding gallons per minute the codes assign each fixture a unit value reflecting its discharge volume and how often it runs; you total the units and read the required drain size from a table. In the IPC the fixture values come from Table 709.1; in the UPC they come from Table 702.1 — and the two codes assign different values to the same fixture, so never mix tables. This calculator uses typical private-use figures for quick totals; verify the values against the plumbing code adopted in your jurisdiction.
What is the difference between DFU and WSFU?
DFU (drainage fixture units) size the sanitary drainage and vent system; WSFU (water supply fixture units) size the water distribution piping. They are separate scales for separate systems, and a fixture can rank very differently on each — a residential clothes washer here counts 2 DFU on the drain side but 4 WSFU on the supply side. The calculator totals both from one fixture list so you can size both systems from the same count.
How many DFU can a 3-inch building drain carry?
Under the IPC building-drain table (Table 710.1) at 1/4" per foot of slope, a 3" building drain carries up to 42 DFU — but any drain serving a water closet must be at least 3" no matter how low the DFU total is. The UPC drainage table (703.2) uses different capacities and pairs with different fixture values, so run the whole calculation inside one code. Verify the size against the drainage tables of the code adopted in your jurisdiction.
Are fixture unit values the same in the IPC and UPC?
No. The two model codes assign different DFU and WSFU values to many of the same fixtures, and both further split values by private versus public use and by details like flush volume. The rule that matters: never average or mix IPC and UPC values in one calculation — total the load and size the pipe entirely inside whichever code your jurisdiction adopts. This calculator uses typical private-use figures, so treat its totals as an estimating starting point rather than a permit submittal.
How do I convert WSFU to GPM?
Through the demand tables, not arithmetic. The conversion follows Hunter’s curve — probable simultaneous demand — so it is strongly nonlinear: 20 WSFU is nowhere near twice the GPM of 10 WSFU. The IPC publishes its conversion in Appendix E (Table E103.3) and the UPC in Chapter 610, and like the fixture values themselves the two are not interchangeable. Total your WSFU here, then read GPM from the table for the code adopted in your jurisdiction.
Method: DFU per IPC Table 709.1, private use (values identical across the 2015–2024 editions); WSFU per IPC Appendix E Table E103.3(2), private “Total” column — note that appendix provisions apply only where the jurisdiction has specifically adopted them (IPC 101.2). Hose bibb and bar sink values are UPC Tables 610.3 / 702.1 (private); the IPC tables carry no row for them. Minimum building-drain size per IPC Table 710.1(1) at 1/4" per foot, with the 3" water-closet floor of note a. IPC and UPC assign different fixture-unit values to the same fixtures — never mix the families — and continuous flows (pumps, hose bibbs in service) count 2 DFU per GPM under IPC 709.3 rather than fixture units. Occupancy (private vs public), the adopted edition, and the AHJ govern the final sizing.
Pair with the DFU chart and WSFU chart for the full IPC and UPC tables, the WSFU to GPM chart to turn the supply total into design flow, or the Pipe Sizing Calculator to carry that flow into a pipe size.