Plumbing Pipe Sizing: How the Hunter Curve and Fixture Units Determine Water Supply Pipe Diameter

A plumber is roughing a two-bathroom house. The plans show a 3/4 inch main feeding the whole building. The inspector asks for the sizing calculation. The plumber knows the pipe is probably oversized, but cannot prove it without running the fixture unit math. Meanwhile, an engineer designing a 20-unit apartment building is sizing the main water service and the old Hunter Curve tables are calling for a 3 inch pipe when field data from similar buildings shows peak flow at less than 40 GPM. Both situations come down to the same calculation: how many fixture units are on the line, and what does the probability curve say about peak demand? Use our Pipe Size and Flow Rate Calculator to calculate minimum pipe sizes from fixture unit counts using Hunter's Curve and IPC/UPC tables.

What Are Water Supply Fixture Units?

Water Supply Fixture Units (WSFU) are a standardized way to express the demand that a plumbing fixture places on a water supply system. The concept was developed by Roy B. Hunter, a physicist at the U.S. National Bureau of Standards, in a 1940 publication that introduced probability-based sizing to plumbing design. Instead of adding up the peak flow rate of every fixture and assuming they all run simultaneously, Hunter's method assigns each fixture a unit value based on its flow rate and usage characteristics, then applies a probability curve to estimate how many fixtures will actually be in use at the same time.

The International Plumbing Code (IPC) Table 604.1 and the Uniform Plumbing Code (UPC) Chapter 6 both use fixture unit values, though the specific numbers differ slightly between the two codes.

Common Fixture Unit Values

Always verify these values against your locally adopted code. Some jurisdictions amend the base code with different values.

The Formula: How Hunter's Curve Works

Hunter's Curve converts total fixture units to peak demand flow rate in gallons per minute (GPM). The relationship is not linear. At low fixture unit counts, the curve is steep because each additional fixture significantly increases the probability of simultaneous use. At higher counts, the curve flattens because the probability of all fixtures running at once decreases.

Key reference points on Hunter's Curve:

Once you have the peak demand GPM, you look up the minimum pipe size in IPC Table 604.3 or UPC Table 610.3, which factors in the developed length of the pipe run and the available pressure at the source.

Step-by-Step Example

A two-bathroom single-family home has the following fixtures:

Using Hunter's Curve, 24.5 WSFU corresponds to approximately 16 GPM peak demand.

Now check IPC Table 604.3. With an available street pressure of 60 psi and a developed length of 80 feet from the meter to the farthest fixture, the minimum pipe size for 16 GPM is 1 inch for the building main. Branch lines to individual bathrooms can be 3/4 inch, and individual fixture risers are 1/2 inch.

If the plumber had installed a 3/4 inch main as originally planned, the friction loss at 16 GPM through 80 feet of 3/4 inch copper would reduce the residual pressure at the farthest fixture below the IPC minimum of 15 psi for flush valve fixtures. The 1 inch main is required.

What Do the Numbers Mean?

The fixture unit system works because of probability. In a home with 24.5 WSFU, the peak demand is 16 GPM, not 40 GPM (which is what you would get by adding all fixtures at full flow). The difference is the statistical reality that not every fixture runs at the same time.

For larger buildings, the probability effect is even more pronounced. The IAPMO Water Demand Calculator, developed in collaboration with the University of Cincinnati and ASPE, is the first major update to peak water demand sizing in over 80 years. It was adopted as Appendix M in the 2018 UPC and has been updated through the 2024 UPC. Ten states have adopted it as an alternative sizing method, including California, Hawaii, Nevada, New Jersey, Oregon, Washington, and Wisconsin.

The IAPMO Water Demand Calculator uses modern probabilistic models that account for low-flow fixtures (1.28 GPF toilets, 1.5 GPM faucets) that did not exist when Hunter developed his curve in 1940. Studies show that the traditional WSFU method overestimates peak demand by 5 to 27 times in multifamily buildings, resulting in pipe diameters much larger than needed. The Water Demand Calculator produces estimates 2 to 6 times the observed peak, which is still conservative but dramatically less wasteful.

Real-World Example

A plumbing engineer is designing the water supply system for a 12-unit apartment building. Each unit has one bathroom with a toilet, lavatory, and shower, plus a kitchen sink and a washing machine hookup. There are also two common-area hose bibbs.

Fixture unit calculation per unit (IPC):

12 units times 9.0 WSFU = 108 WSFU. Plus 2 hose bibbs at 2.5 each = 5 WSFU. Building total: 113 WSFU.

Using Hunter's Curve, 113 WSFU corresponds to approximately 52 GPM peak demand. With 60 psi street pressure and a 120-foot developed length, IPC Table 604.3 requires a 2 inch building main.

Using the IAPMO Water Demand Calculator with the same fixture count and modern low-flow fixtures (1.28 GPF toilets, 1.5 GPM faucets, 2.0 GPM showers), the calculated peak demand is approximately 22 GPM. At 22 GPM with the same pressure and length, a 1.5 inch main would be sufficient. The difference between 2 inch and 1.5 inch pipe is not just material cost. It also means longer wait times for hot water, greater water stagnation in oversized pipes, and increased risk of water quality issues.

The engineer checks which code the jurisdiction has adopted. If the 2024 UPC with Appendix M is in effect, the Water Demand Calculator can be used. If only the traditional method is accepted, the 2 inch main is required by code even though it is significantly oversized for the actual demand.

Common Mistakes to Avoid

Using IPC fixture unit values in a UPC jurisdiction. The values are similar but not identical. A toilet is 2.5 WSFU under both codes, but the drainage fixture unit differs: 3.0 DFU under IPC versus 4.0 DFU under UPC. Using the wrong table can result in undersized drain lines or oversized supply pipes. Always confirm which code your jurisdiction has adopted before starting the calculation.

Not separating hot and cold water fixture unit calculations. Hot water demand is typically 60 to 75 percent of total water demand. Calculate hot and cold fixture units separately so the hot water main can be sized smaller than the cold water main. This reduces material cost and improves hot water delivery time.

Assuming all fixtures run at full demand. Hunter's Curve accounts for the probability that not all fixtures operate simultaneously. 100 fixture units does not equal 100 GPM. The curve flattens at higher counts. A common error is to add all fixture flow rates together and size the pipe for that total, which produces massively oversized piping.

Ignoring available pressure and developed length. Pipe sizing is not determined by fixture units alone. IPC Table 604.3 and UPC Table 610.3 require the developed length of the pipe run and the available pressure at the source. A 50 WSFU load with 80 psi street pressure and a 50-foot run may only need a 1 inch pipe. The same load with 35 psi and a 200-foot run may need 1.5 inches. Always include pressure and length in the sizing lookup.

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Frequently Asked Questions

What is the Hunter Curve in plumbing?

The Hunter Curve is a probability-based method for estimating peak water demand in buildings. Developed by Roy B. Hunter in 1940, it converts total water supply fixture units (WSFU) to an estimated peak flow rate in GPM. The curve accounts for the statistical reality that not all fixtures operate simultaneously, preventing both undersized and oversized piping.

What is the difference between WSFU and DFU?

WSFU (Water Supply Fixture Units) measures demand on the water supply system. DFU (Drainage Fixture Units) measures load on the drainage system. The two use different values for the same fixture. A toilet is 2.5 WSFU but 3.0 DFU under IPC (4.0 DFU under UPC). Calculate supply and drainage sizing separately.

How accurate is the Hunter Curve for modern buildings?

The Hunter Curve tends to overestimate peak demand in modern buildings because it was developed before low-flow fixtures existed. Studies show overestimation by 5 to 27 times in multifamily buildings. The IAPMO Water Demand Calculator, adopted in 10 states as an alternative method, uses updated probability models that produce estimates 2 to 6 times actual peak demand, which is still conservative but significantly less wasteful.

What pipe size do I need for a residential bathroom group?

A typical bathroom group (toilet, lavatory, and shower) has approximately 5.5 WSFU under IPC. Using Hunter's Curve, this corresponds to about 6 GPM peak demand. For a typical residential run of 30 to 50 feet with 40 to 60 psi available pressure, a 3/4 inch branch line is sufficient. Individual fixture risers should be 1/2 inch.

Can I use the IAPMO Water Demand Calculator instead of Hunter's Curve?

It depends on your jurisdiction. Ten states have adopted the Water Demand Calculator as an alternative sizing method: California, Hawaii, Montana, Nevada, New Jersey, New Mexico, North Dakota, Oregon, Washington, and Wisconsin. If your jurisdiction has adopted the 2024 UPC with Appendix M, you can use the Water Demand Calculator. Otherwise, you must use the traditional Hunter Curve method.

Conclusion

Pipe sizing starts with fixture unit counts and ends with a code table lookup that factors in available pressure and developed length. The Hunter Curve has served the plumbing industry for over 80 years, but it overestimates demand in modern buildings with low-flow fixtures. If your jurisdiction has adopted the IAPMO Water Demand Calculator, use it to avoid oversized piping, reduce material costs, and improve water quality. If not, the traditional method still applies. Either way, calculate fixture units separately for hot and cold water, verify which code your jurisdiction follows, and always include pressure and pipe length in your sizing lookup. The Pipe Size and Flow Rate Calculator handles the fixture unit to GPM conversion and pipe size selection automatically.