Every plumbing system in America is sized using a method developed in the 1940s by a researcher named Roy Hunter at the National Bureau of Standards. Hunter's work introduced the concept of fixture units, a way to assign a single number to each type of plumbing fixture that represents its probable contribution to peak water demand. These fixture unit values are then summed and converted to gallons per minute using a probability curve known as Hunter's Curve.
The method works because not every fixture runs simultaneously. A building with 50 fixtures will never have all 50 open at once. Hunter used probability theory to estimate the likely peak demand based on the number and type of fixtures, their individual flow rates, and the probability of simultaneous use. The result is a design flow rate that is a fraction of the theoretical maximum but sufficient to serve the building under normal peak conditions.
Understanding fixture units is essential for anyone who sizes plumbing systems, reviews plumbing plans, or needs to determine whether existing infrastructure can support additional fixtures during a renovation.
What Is a Water Supply Fixture Unit?
A water supply fixture unit (WSFU) is a dimensionless number assigned to each type of plumbing fixture. It represents the fixture's load-producing effect on the water supply system in terms of both flow rate and duration. Higher WSFU values mean the fixture draws more water, draws for longer, or both.
Standard WSFU values from IPC Table 604.3: a private lavatory faucet is 1.0 WSFU, a flush-tank water closet is 2.5 WSFU (private) or 2.5 WSFU (public), a flush-valve water closet is 5.0 WSFU (much higher because flush valves draw a large flow for a short burst), a bathtub is 4.0 WSFU, a shower is 2.0 WSFU, a kitchen sink is 1.5 WSFU, and a clothes washer is 4.0 WSFU.
The values differ between IPC and UPC, and between private (residential) and public (commercial) use. Public fixtures generally have higher WSFU values because they are used more frequently. The distinction matters for mixed-use buildings and code compliance.
The WSFU values were calibrated to the fixtures of the 1940s, which used much more water than modern low-flow fixtures. A 1940s toilet used 5-7 gallons per flush; today's standard is 1.28-1.6 GPF. Despite this, the WSFU values have not been substantially updated in most codes. This means the method overpredicts demand for modern buildings, which is conservative but expensive in terms of oversized piping.
Lavatory: 1.0
Water closet (flush tank): 2.5
Bathtub: 4.0
Shower: 2.0
Kitchen sink: 1.5
Clothes washer: 4.0
Dishwasher: 1.5
Fixture Unit Calculator
Calculate water supply fixture units (WSFU) per IPC/UPC and convert to peak GPM demand using Hunter's Curve. Determine minimum pipe size for water supply systems.
Hunter's Curve: Converting WSFU to GPM
Once you have totaled the WSFU for the building, you convert to gallons per minute using Hunter's Curve, a tabulated relationship that accounts for the probability of simultaneous use. The relationship is nonlinear: doubling the WSFU does not double the GPM demand. At 10 WSFU, the estimated demand is about 8 GPM. At 100 WSFU, it is about 38 GPM. At 1,000 WSFU, it is about 170 GPM. The curve flattens because the probability of a very large fraction of fixtures running simultaneously is extremely low.
There are two versions of Hunter's Curve: one for systems with predominantly flush-tank fixtures and one for systems with predominantly flush-valve fixtures. Flush-valve systems produce higher peak demand because flush valves draw 15-30 GPM during the flush cycle, compared to 3-5 GPM for a flush-tank toilet. Most residential systems use the flush-tank curve. Commercial buildings with flush-valve fixtures use the flush-valve curve.
The IPC provides the conversion in Table 604.4. The UPC provides it in Table A-1. Both tables are based on Hunter's original research with minor adjustments. To use the table, look up your total WSFU in the left column and read the corresponding GPM in the right column.
The converted GPM is your design flow rate for sizing the water service, meter, and distribution piping. It represents the peak demand that the system must be able to deliver without excessive pressure loss or noise.
10 WSFU = ~8 GPM
20 WSFU = ~15 GPM
50 WSFU = ~25 GPM
100 WSFU = ~38 GPM
200 WSFU = ~65 GPM
500 WSFU = ~120 GPM
Note the diminishing marginal GPM per WSFU as total increases.
From GPM to Pipe Size: The Pressure Budget Method
Once you have the design GPM, you need to select a pipe diameter that delivers that flow with adequate pressure at the most remote fixture. The method uses a pressure budget: start with the available street pressure and subtract all the losses between the street and the fixture.
Pressure budget: Available pressure = street pressure minus static head loss (0.433 PSI per foot of elevation) minus meter loss minus backflow preventer loss minus friction loss in piping. The remaining pressure must be at least 8 PSI at the most remote fixture (15 PSI for flush-valve fixtures).
Friction loss depends on the pipe diameter, flow rate, and pipe material. Smaller pipe at the same flow rate produces more friction. The IPC provides friction loss charts in Appendix E. The goal is to select the smallest pipe diameter that keeps friction loss within the pressure budget while delivering the design GPM.
In practice, residential water service pipes are 3/4-inch or 1-inch for most homes. 3/4-inch service handles up to about 12-15 GPM. 1-inch service handles up to about 25-30 GPM. For larger homes with high fixture counts or long runs from the street, 1-1/4 or 1-1/2 inch service may be needed.
P_available = P_street - P_elevation - P_meter - P_backflow - P_friction
P_elevation = 0.433 PSI per foot of height above service entry
P_fixture_min = 8 PSI (flush tank) or 15 PSI (flush valve)
P_available must exceed P_fixture_min at the most remote fixture.
Why Hunter's Curve Overpredicts for Modern Buildings
Hunter's Curve was calibrated to 1940s fixtures: 5-7 GPF toilets, 5 GPM showerheads, and 3 GPM faucets. Modern low-flow fixtures use dramatically less water: 1.28-1.6 GPF toilets, 2.0-2.5 GPM showerheads, and 0.5-1.5 GPM faucets. Studies by researchers at the Stevens Institute of Technology and others have shown that actual peak demand in modern residential and commercial buildings is 20-40% below what Hunter's Curve predicts.
The IAPMO (UPC) and ICC (IPC) code committees have been slow to update the tables because reducing pipe sizes raises the stakes if the new tables underpredict demand. Overprediction is safe but expensive. Underprediction causes inadequate flow and pressure complaints. The conservative approach has been to leave the tables as-is and let engineers use professional judgment to adjust.
Some progressive jurisdictions and the latest versions of the IPC have introduced modified fixture unit values for WaterSense-rated fixtures. If your code allows it, using modified values can reduce pipe sizes and save material cost. If not, size to the standard tables and know that you have a built-in safety margin.
For renovation projects where you are adding fixtures to an existing building, the conservatism of Hunter's Curve works in your favor. The existing piping was sized for 1940s-era fixtures. If you are replacing old fixtures with low-flow models and adding a few more, the total demand may actually decrease even as the fixture count increases.