Fixture Unit Calculator
Totals Water Supply Fixture Units (WSFU) and Drainage Fixture Units (DFU) for any building. Uses IPC Table 604.4 and Table 709.1 to convert to GPM and pipe sizes.
Enter Fixture Quantities
| Fixture | WSFU | DFU | Qty |
|---|---|---|---|
| Bathtub | 4 | 2 | |
| Shower | 4 | 2 | |
| Lavatory (bathroom sink) | 2 | 1 | |
| Kitchen Sink | 3 | 2 | |
| Dishwasher | 2 | 2 | |
| Clothes Washer | 4 | 3 | |
| Toilet (flush tank) | 5 | 4 | |
| Toilet (flush valve) | 10 | 4 | |
| Hose Bibb | 5 | 0 | |
| Utility Sink | 3 | 2 | |
| Bar Sink | 2 | 1 | |
| Floor Drain | 0 | 2 |
Results
Values per IPC Tables 604.4, 709.1, and E103.3. Pipe sizes are for building main — branch sizing may differ.
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How to Use This Calculator
1. Count each fixture type. Walk through the building or review the plans. Count every plumbing fixture that will be connected — supply side and drain side.
2. Enter quantities in the table. Each fixture type has pre-loaded WSFU and DFU values from IPC Tables 604.4 and 709.1. Just enter how many of each fixture you have.
3. Note the toilet type. Flush tank toilets (residential) are 5 WSFU. Flush valve toilets (commercial) are 10 WSFU. Using the wrong type will significantly change the supply pipe sizing.
4. Read the results. The calculator totals WSFU and DFU, converts to GPM, and recommends both supply and drain pipe sizes for the building main.
How the Fixture Unit Formula Works
Fixture units are a standardized way to express plumbing demand without calculating each fixture individually. The IPC tables convert these totals into pipe sizes.
Total WSFU = Sum of (each fixture's WSFU x quantity)
Total DFU = Sum of (each fixture's DFU x quantity)
GPM from Hunter's Curve (IPC Table E103.3):
1-2 WSFU=3 GPM, 3-5=5, 6-10=8, 11-20=14, 21-40=22
41-60=29, 61-100=38, 101-200=55, 201-400=80
Supply pipe: based on WSFU and available pressure
Drain pipe: <24 DFU=3", 24-42=3", 42-216=4", 216+=6"
Hunter's Curve accounts for diversity — the statistical probability that all fixtures will not be used simultaneously. This is why 100 WSFU converts to only 38 GPM rather than the theoretical sum of individual flows.
When To Use This
Building main sizing. Total all fixtures in the building to determine the water service pipe and meter size. This is the first step in any plumbing system design.
Addition or remodel impact. Adding fixtures increases demand. Calculate current fixture units, add the new ones, and check if the existing main and drain are large enough.
Permit applications. Most jurisdictions require a fixture unit count as part of the plumbing permit submittal. This calculator gives you the totals and pipe sizes to include.
Frequently Asked Questions
What are Water Supply Fixture Units (WSFU)?
What is the difference between WSFU and DFU?
How do I convert fixture units to GPM?
What size drain pipe do I need based on DFU?
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