Water Supply Fixture Unit (WSFU): Sizing the Water Service and Meter

What a water supply fixture unit measures

Every fixture pulls water when it is used, but not all at once. A whole-house water line sized for every tap running at full flow would be far too big.

A WSFU gives each fixture a weight that reflects three things: how fast it draws water, how long it runs, and how often it is used ³.

The concept comes from Roy B. Hunter’s 1940 report for the National Bureau of Standards. He assigned weights such as 10 for a public flush-valve toilet, 5 for a flush tank and 4 for a bathtub, and suggested home bathrooms get half the weight of the same fixtures in busy public use ².

The Copper Development Association’s handbook explains the practical use: add the fixture unit values for all fixtures on a pipe, then translate that total into expected maximum demand using the procedure in your local code. It also notes that assigned values vary by jurisdiction ³.

How a WSFU total becomes a pipe size

1. Add the fixture units. Use your code’s table for each fixture or group, hot, cold or combined as needed ¹.
2. Pick the right curve. Systems mostly on flush tanks and systems mostly on flushometer valves convert differently ¹.
3. Convert to gpm. Read the probable demand from the code table; 10 WSFU on a flush-tank system is 14.6 gpm in the 2012 IRC table ¹.
4. Add continuous demand. Hose bibbs, lawn sprinklers and air conditioning that run steadily are added in gpm ³².
5. Work out pressure available. Street or well pressure, minus meter loss, minus 0.434 psi for each foot of rise, minus pressure needed at the fixture ³.
6. Choose the size. Pick a pipe whose friction loss over its length fits the pressure budget and whose velocity stays under limits, such as 8 ft/s cold and 5 ft/s hot for copper ³.

How many WSFUs home fixtures and room groups carry (2012 IRC model table)

Example: estimating peak demand for a two-bath house

This uses the 2012 IRC model values and its flush-tank conversion table to estimate a home’s peak demand. The house is an illustration; the values are from the table.

Compare that with adding every fixture’s full flow, which would be far higher; the table builds in the low chance of everything running at once. The next steps are a pressure budget and pipe sizing.

If a lawn sprinkler system will run during peak use, the CDA handbook says to add its actual gpm on top, since fixture-unit curves cover intermittent use only ³. The fixture unit calculator turns a WSFU total into demand using Hunter’s public curves.

Where WSFU totals show up in a project

On a new home or addition, the plumber or designer totals WSFUs to size the water service from the street or well and the main distribution lines. Utilities may ask for a fixture count or WSFU total when you apply for a new or larger water meter.

In a remodel, adding a bathroom, a second kitchen or an ADU adds WSFUs to an existing service. If the service and meter were sized tightly, showers may lose pressure when other fixtures run. Our repiping service page covers when supply lines get upsized.

In the California code, the text says the WSFU values in Table 610.3 include both hot and cold demand ⁵. Some IRC-style tables split hot, cold and combined values, which lets designers size hot and cold branches separately ¹.

The type of toilet matters as much as the count. The same IRC table has a second column for systems mostly on flushometer valves, and at low totals it calls for far more flow than the flush-tank column in the chart above. The four pairs below are read straight from that table.

What WSFUs do well and where they fall short

What a WSFU total does not tell you

It is not a pipe size by itself. Hunter wrote that the weight scale is not a definite unit of flow but a load factor applied through a relation between fixtures and demand ². The gpm figure still has to be checked against pressure, pipe length, elevation and velocity ³.

It does not size drains. That is a separate table and a separate check.

It may overstate demand in modern homes with low-flow fixtures. IAPMO’s Water Demand Calculator was built to estimate peak demand for single-family and multifamily homes with today’s efficient fixtures, and it keeps at least a 2x safety factor over measured use ⁴. Where your code allows it, it can be an alternative.

How WSFUs affect the cost of a water service

We do not list prices; fair numbers need dated local quotes.

A higher WSFU total can raise the size of the water service, the meter and the main lines.

Larger meters often carry higher utility connection or capacity charges, and larger pipe costs more to buy and install.

Long runs, uphill lots and low street pressure push sizes up further, because the pressure budget gets tighter ³.

On the other side, IAPMO reports an average saving of 513 gallons per dwelling unit per year from right-sizing with its calculator ⁴. See our plumbing cost guide and repipe cost estimator for how supply work is priced.

How a supply system is sized on a permitted project

Supply sizing is permitted work, done by a licensed plumber or designer. The general sequence:

1. Confirm code and edition. UPC-based codes like the CPC, IPC-based codes, or an approved alternative such as the IAPMO calculator ⁴.
2. Get the street or well pressure. The 2012 IRC model text required at least 40 psi static at the building entrance and no more than 80 psi without a pressure-reducing valve ¹.
3. Total WSFUs. By fixture or group, for each pipe segment ¹.
4. Convert and add continuous loads. Table or curve, plus sprinklers and similar in gpm ³.
5. Run the pressure budget. Subtract meter loss, elevation at 0.434 psi per foot and fixture pressure ³.
6. Size pipes and meter. Within friction and velocity limits; the 2012 IRC model text set a ¾-inch minimum water service ¹.
7. Inspect and test. Then the supply system gets its pressure test.

How to keep supply capacity where it was designed

• Before adding fixtures or an ADU, ask whether the service and meter have room.
• Keep a record of the original fixture count or permit plans.
• Watch for pressure drops when two fixtures run; that can be a sizing limit, a failing pressure-reducing valve or scaled pipe.
• Clean aerators and showerheads before blaming the pipes.
• Old galvanized pipe narrows inside over time, lowering capacity no table accounts for.

Which signs mean your supply is undersized

• Shower pressure collapses when another fixture runs.
• Upstairs fixtures are weak while downstairs ones are fine.
• Noise or water hammer at high flow; fast water is noisy, which is why velocity limits exist ³.
• Pinhole leaks in copper near elbows, which high velocity can contribute to through erosion-corrosion ³.
• Pressure that has fallen since a remodel added fixtures.

A plumber can measure static and flowing pressure and check pipe sizes. Static pressure is read with every tap closed and flowing pressure with a fixture open; a large gap between them points to the pipe. The leak detection service covers hidden leaks that also sap pressure.

Which codes and methods use WSFUs

California Plumbing Code, 2025 edition (UPC-based, in force 1 January 2026), §610.3: fixture water demand is represented by fixture units in Table 610.3, which include both hot and cold demand ⁵.

§610.4 (2022 text, not re-read in 2025): systems within Table 610.4 may be sized from that table or by the Appendix method in §610.5 ⁶. Neither public copy showed the table values, so they are not reproduced here.

International Residential Code, 2012 model text (P2903.6 and Tables P2903.6 and P2903.6(1)): supply loads are figured by total WSFU on the pipe being sized and converted to gpm; minimum ¾-inch water service; 40 psi minimum and 80 psi maximum static pressure ¹. Later IRC editions may differ; check your adopted edition.

IAPMO Water Demand Calculator: Appendix M of the Uniform Plumbing Code and Chapter 5 of the 2023 WE-Stand; IAPMO lists California’s statewide adoption for multifamily hot water systems and several local adopters ⁴.

Hunter, NBS BMS65 (1940): the public-domain origin of fixture weights and demand curves ².

How WSFUs compare with DFUs, flow rate and the IAPMO calculator

What people get wrong about WSFUs

Myth One WSFU equals one gallon per minute.

Reality In the 2012 IRC table, 10 WSFU on flush tanks is 14.6 gpm and 50 is 29.1 gpm; the relation is not one to one ¹.

Myth Double the fixtures, double the pipe.

Reality Demand grows more slowly than fixture units because of diversity ¹³.

Myth WSFUs cover the sprinkler system.

Reality Continuous loads are added in gpm on top ³.

Myth Bigger pipe is always safer.

Reality IAPMO says oversizing raises costs and stagnation concerns ⁴.

Myth Any code’s WSFU table will do.

Reality Values vary by jurisdiction; use the adopted code ³.

How your state code and building site change the answer

When WSFUs matter to you and what to ask

They matter when you build, add a bathroom or ADU, apply for a larger water meter, replace a water service line, or fight low pressure that started after a remodel.

Your next step: list every fixture and outdoor use you plan, including irrigation, and ask your plumber to confirm the service and meter size against your adopted code. If a utility quotes a larger meter, ask for the WSFU total and table behind it.

Related terms

Drainage Fixture Unit (DFU)

A drainage fixture unit (DFU) is a code-assigned number that rates how much load one plumbing fixture puts on a drain, so the units from every fixture on a pipe can be added up and checked against the largest load that pipe size is allowed to carry.

Plumbing Flow Rate

Plumbing flow rate is the volume of water that moves through a pipe or out of a fixture in a set time, usually stated in gallons per minute (gpm), and it is what decides how fast a tub fills or how strong a shower feels.

Residential Water Pressure

Residential water pressure is the force that pushes water through a home’s supply pipes, measured in pounds per square inch (psi), usually read as static pressure with no water running.

Water Meter

A water meter is the device, usually in a box near the curb or in a basement, that measures the volume of water flowing from the utility’s pipe into a property, so the utility can bill for it and you can track use and spot leaks.

Potable Water System

A potable water system is the pressurized network of pipes, valves and fixtures that carries water safe to drink from a public main or private well to every tap in a building, kept separate from drains and any non-drinking water.

Pressure-Reducing Valve (PRV)

A pressure-reducing valve (PRV) is a spring-and-diaphragm valve on a home’s incoming water line that automatically lowers high, changing street pressure to a steady, adjustable set point, so pipes, fixtures and appliances inside the house are not exposed to excess pressure.

PEX Plumbing Pipe

PEX plumbing pipe is flexible water supply tubing made from cross-linked polyethylene, a plastic whose molecules are chemically bonded to each other so it holds up to hot, pressurized, chlorinated drinking water.

Copper Plumbing Pipe

Copper plumbing pipe is round tube made of at least 99.9 percent copper, sold in hard and soft tempers and in wall thicknesses called Types K, L and M, that carries drinking water through homes and is joined with solder, brazing, flare, press or push fittings.

Also referenced from: Water Hammer, Plumbing Flow Rate, Plumbing Rough-In, Drainage Fixture Unit (DFU)