WSFU Calculator: Water Supply Fixture Units to GPM

What Is a Water Supply Fixture Unit?

A Water Supply Fixture Unit, or WSFU, represents the probable demand a plumbing fixture places on the water-supply system. It is not a direct flow rate. Instead, fixture units provide a common way to combine many different fixtures into a probable demand calculation.

A water closet, lavatory, shower, bathtub, kitchen sink, dishwasher, clothes washer, and other fixtures can each contribute WSFU to the system. Fixtures with different operating characteristics can have different fixture-unit values even if their connection sizes are similar.

WSFU vs DFU: What's the Difference?

WSFU and DFU measure different sides of a plumbing system. Water Supply Fixture Units estimate probable demand on the water-supply system. Drainage Fixture Units estimate probable discharge load on the drainage system.

Cold, Hot, and Combined WSFU

Fixtures supplied with both hot and cold water can have separate cold, hot, and combined fixture-unit values. These are used for different portions of the water-supply system.

• Cold WSFU is useful when evaluating the cold-water piping serving the fixture group.
• Hot WSFU is useful when evaluating the hot-water distribution load.
• Combined WSFU represents the probable combined demand and is not necessarily equal to cold WSFU plus hot WSFU.

How WSFU Converts to GPM

WSFU does not convert to gallons per minute using a simple fixed multiplier. Plumbing demand methods account for the probability that not every fixture will operate simultaneously.

The calculator uses a tabulated demand relationship to estimate probable peak flow from the combined WSFU total. The applicable demand curve depends in part on whether the system predominantly uses flush-tank fixtures or flushometer valves.

Example Residential WSFU Calculation

Consider a house with two private flush-tank water closets, two lavatories, two shower heads, one kitchen sink, one dishwasher, and one clothes washer.

That probable demand is only one input to final water-supply sizing. Pressure, developed length, elevation, meter losses, pipe material, and local code requirements still need to be checked.

Example: Existing House Plus ADU

If an existing house and a new ADU share the same water service, the demand on the shared portion of the system should be reviewed using the fixtures served by that common piping. The ADU demand should not be evaluated in isolation when both buildings rely on the same service.

The larger fixture-unit total does not automatically determine one pipe diameter. The shared service still needs enough available pressure after accounting for developed length, elevation, meter and regulator losses, pipe material, and applicable code requirements.

WSFU Does Not Determine Pipe Size by Itself

A WSFU total is an important input to water-supply sizing, but it does not by itself tell you whether a pipe should be 1/2 inch, 3/4 inch, 1 inch, or another size.

Final water pipe sizing can depend on:

• Available static and residual water pressure
• Total developed piping length and resulting friction loss
• Elevation gain or loss and its effect on available pressure
• Water meter size and pressure loss
• Backflow preventer or pressure regulator losses
• Pipe material and inside diameter
• Allowable velocity and friction loss
• Required pressure at the most remote fixture
• Continuous water demands
• Adopted plumbing code and local amendments

Developed length and elevation affect how much of the available water pressure remains at the fixtures. Longer piping creates additional friction loss. An increase in elevation also reduces available pressure by approximately 0.433 psi for each foot of vertical rise. Because of these losses, two systems with the same WSFU demand can require different pipe sizes when one has a substantially longer run or serves fixtures at a higher elevation.

Continuous and Known Water Demand

Not every water demand should be treated as an ordinary plumbing fixture unit. Irrigation, known hose bibb use, process equipment, cooling equipment, and other continuous-use or known-flow loads may need to be accounted for using their actual flow demand rather than assigning an arbitrary fixture-unit value.

The calculator therefore includes a separate additional continuous or known demand input in GPM. When applicable, that flow is added to the estimated probable fixture demand to provide a planning demand for the next stage of water-supply sizing.

The actual amount entered should come from the equipment demand, irrigation design, known outlet flow, or other project-specific information rather than from an assumed WSFU conversion.

UPC vs IPC Water-Supply Fixture Units

Plumbing codes can use different fixture-unit values and different water-supply sizing methods. This calculator uses IPC-style water-supply fixture-unit values and demand relationships for planning purposes rather than mixing values from multiple code systems.

Review the UPC vs IPC plumbing code comparison and the state plumbing code lookup before relying on a fixture-unit calculation for final design.

How to Use the WSFU Calculator

1. Identify every fixture served by the pipe section.
2. Enter the quantity of each fixture.
3. Review the calculated cold, hot, and combined WSFU totals.
4. Select the demand curve that best matches the overall fixture system.
5. Review the estimated probable fixture demand in GPM.
6. Enter any known continuous or additional demand in GPM when applicable.
7. Use the resulting planning demand as one input to the full water-supply sizing process.
8. Verify fixture values, demand method, pressure, developed length, elevation, and final sizing requirements with the adopted plumbing code and local authority.

Frequently Asked Questions