One fixture, two systems
WSFU and DFU Are Separate Planning Totals
WSFU describes the likely demand a fixture places on the incoming water supply. DFU describes the probable wastewater load it sends to the sanitary drain. A fixture can add to both totals, but one is never converted into the other.
Use WSFU with the project’s water-demand method. Use DFU with the capacity table for the drain section. Keeping those paths separate prevents a supply decision from being mistaken for a drainage decision.
Add a fixture to show the active WSFU and DFU values from the selected code record.
Water supply
WSFU Helps Start the Water-Supply Plan
WSFU is a way to add the likely water use of the fixtures served by the same supply section. A shower, lavatory, toilet, or sink contributes a code value to that total, based on its type and how it is used.
The total is not a GPM reading and it does not name a pipe size. Use it with the demand method accepted for the project to find a design flow, then use that flow when you size the supply pipe.
Sanitary drainage
DFU Helps Check the Drainage Path
DFU is a separate way to add the wastewater load from fixtures that drain to the same branch, stack, or building drain. It uses drainage values from the selected code, not the water-supply values above.
Use the DFU total with the capacity table for the drain section you are checking. That later check needs the right pipe application and, for horizontal drains, the applicable slope. DFU is not a GPM value or a supply-pipe calculation.
Worked schedule
Example: From Fixture List to WSFU and DFU
This simplified private-use schedule is for a shared building or zone schedule serving a qualifying bathroom group and a kitchen sink. It is not a bathroom-branch example unless that branch really serves both.
Water-supply path
Step 1: Add the water-supply values
Under the 2024 IPC private-use entries, one qualifying bathroom group contributes 3.6 WSFU and one domestic kitchen sink contributes 1.4 WSFU. Use the bathroom-group row only when the fixture mix matches that group definition.
3.6 + 1.4 = 5 WSFU
What happens next: 5 WSFU is not 5 GPM. Put the WSFU total into the demand method accepted for the project. That method gives the design flow used to choose a supply-pipe size.
Sanitary-drain path
Step 2: Add the drainage values
For the same fixture list, the qualifying bathroom group contributes 5 DFU and the kitchen sink contributes 2 DFU. These are drainage values, so they are added separately from the WSFU total.
5 + 2 = 7 DFU
What happens next: Match 7 DFU to the capacity table for the exact drain section, such as a horizontal branch or building drain. After the drain size and rule are known, calculate the required fall for a horizontal run.
Use each result for its own job
What to Do With Your WSFU and DFU Totals
Use WSFU to plan the water supply
Use WSFU with the supply-demand method accepted for the project. It is not a direct flow reading and does not choose a pipe on its own.
Use DFU with the correct capacity table
Match DFU to the code table for the exact branch, stack, or building drain you are sizing. That check also needs the relevant slope and fixture limits.
Do not treat fixture units as GPM
WSFU can become design flow only through the accepted demand method. DFU is never a GPM value.
Choose the right fixture row
Why the Exact Fixture Matters
A code change, occupancy change, or toilet type can change the value before quantity is even counted.
IPC and UPC are different lists
They do not always give a fixture the same value. When you change the code, the calculator changes the values too, not just the name at the top.
Private and public use can differ
A fixture in a home may have a different value from the same fixture in a public restroom. Check each row when the project has both.
Toilets are not all the same
A toilet with a tank and one with a flushometer valve can use different supply values. Pick the type shown on the plan or product information.
Sources and limits
What This Fixture Calculator Totals and What It Does Not Size
Data sources last checked September 28, 2026.
| What you need to know | How this calculator helps |
|---|---|
| What it does | Adds the fixture values you select as WSFU, DFU, or both, and shows the selected code basis and source for each row. |
| What it does not do | It does not choose a supply pipe, drain pipe, vent, drain slope, water heater, or actual flow. It also does not confirm occupancy classification, local amendments, fixture approvals, or whether a bathroom-group shortcut applies. |
| Project rule | Use the code and local changes adopted for the project. IPC supply values on this page use Appendix E only where the local jurisdiction accepts or refers to it. |
| Source | How it is used here |
|---|---|
| International Code Council - 2024 IPC | Appendix E supply basis and Table 709.1 drainage basis. |
| IAPMO - Read the 2024 UPC Online | Table 610.3 supply and Table 702.1 drainage bases. |
Quick answers
Plumbing Fixture Calculator FAQs
Which fixtures should I add for one branch?
Add only the fixtures that branch serves. For example, a restroom branch includes that restroom’s fixtures. Do not add fixtures on another branch or before the branch starts.
Can I add a bathroom group and the fixtures inside it?
Usually no. The group row already includes the fixtures named in that group. Add individual rows only for fixtures outside that group.
Why do I need to choose the toilet type?
A tank toilet and a flushometer toilet can have different supply values. Private and public use can change the value too. Pick the row that matches the fixture on the plan.
What if my local code uses different values?
Choose the code used for the project. If a local rule or approved schedule has different values, use the custom/local option and add the source note for that value.
What if my fixture is not listed?
Use the custom fixture-unit row. Enter the WSFU and/or DFU values from the adopted project schedule and include the source/reference note required by the calculator.
Why does a fixture show Needs review?
The selected code and use category do not have a verified record on this page, or the fixture needs a different code-specific method. Choose a supported record, use Drainage Only when appropriate, or add a sourced custom row.
What should I do with the WSFU answer?
Use WSFU with the supply-demand method accepted for the project to establish design flow. Use the Pipe Size Calculator only after you know that flow.
What should I do with the DFU answer?
Check DFU against the capacity table for the branch, stack, or building drain you are sizing, including its slope and water-closet limits. Use the Drain Pipe Slope Calculator to work out the fall once the drain size and criterion are known.
Why is there no GPM answer?
WSFU and DFU are not direct flow readings. A measured outlet flow also does not establish whole-building demand. Use the Water Flow Rate Calculator when you need to measure one tap or pipe.