IPC 604.3 · 604.4 · Water supply sizing

How water supply pipe sizing works

Fixture pressure and flow criteria, maximum flow rates, water supply fixture units, and the pressure-loss (friction budget) method — with worked examples and a free link to the official tables.

Need the actual tables? Read IPC Section 604 and Appendix E free at codes.iccsafe.org →

What each IPC water-supply table is for

Table / section	What it tells you	When you use it
Table 604.3	The minimum flow and the minimum flowing pressure each fixture needs at its supply outlet	Sets the pressure you must still have at the most demanding fixture after all losses
Table 604.4	The maximum flow rate or flush volume allowed for faucets, showerheads, water closets, and urinals	Water-conservation limits — also why modern demand is lower than the old tables assumed
604.5	Minimum size of the individual fixture supply pipe	The short pipe from the branch to each fixture
604.8	When a pressure-reducing valve is required	High street or well pressure
Appendix E (WSFU tables)	Fixture-unit values per fixture and a conversion from total fixture units to design flow (gpm)	Turning a list of fixtures into a peak demand
Appendix E friction-loss charts	Pressure loss per 100 ft for each pipe material and size at a given flow	Picking the pipe size that fits your friction budget

Our own summary, not the code text. Read IPC Section 604 and Appendix E free at codes.iccsafe.org (2021/2024 IPC) and verify against the edition adopted by your AHJ. Not engineering advice — larger systems are designed by a licensed engineer.

The pressure-loss method, step by step

Step	What you do	Watch out for
1	List every fixture, assign its water supply fixture units (WSFU), and total them for each pipe section	Private and public fixtures carry different values; hot and cold are listed separately from the combined value
2	Convert total WSFU to design flow in gpm using the Appendix E demand table	Flush-tank and flushometer systems use different curves
3	Start from the minimum static pressure at the main (ask the water utility)	Use the low-season, low-pressure figure, not a single gauge reading
4	Subtract elevation: 0.433 psi for every foot the highest fixture sits above the main	That is physics, not code — it applies to every system
5	Subtract the meter, backflow preventer, softener, and filter losses at design flow (manufacturer data)	Reduced-pressure backflow assemblies can eat a big share of the budget
6	Subtract the minimum flowing pressure the most demanding fixture needs (Table 604.3)	The highest fixture is not always the most demanding one
7	What is left is the friction budget. Divide it by the equivalent length (developed length plus an allowance for fittings) and express it per 100 ft	Appendix E notes a common rule of adding about half again to developed length for fittings
8	Pick the smallest pipe whose friction loss at design flow stays under the budget and whose velocity stays within your design limit	Velocity limits protect copper from erosion and cut water hammer

Worked example — friction budget for a small commercial building

Input / step	Value	Source
Minimum static pressure at the main	60 psi	Utility (assumed)
Highest fixture 25 ft above the main	− 10.8 psi	25 × 0.433
Meter loss at design flow	− 6 psi	Manufacturer curve (assumed)
Backflow preventer loss at design flow	− 12 psi	Manufacturer curve (assumed)
Minimum pressure at the highest flushometer fixture	− 15 psi	Appendix E example value for flushometer valves
Friction budget	16.2 psi	60 − 10.8 − 6 − 12 − 15
Developed length 120 ft × 1.5 fitting allowance	180 ft equivalent	Appendix E rule of thumb
Allowable friction	≈ 9.0 psi per 100 ft	16.2 × 100 ÷ 180
Next	Find the design gpm from total WSFU, then the smallest pipe on the friction chart at or under 9.0 psi/100 ft	Appendix E

Worked example values only (assumed site data). Read Section 604 and Appendix E free at codes.iccsafe.org.

Worked example — reading a fixture-unit value

Input / step	Value	Source
Fixture	Water closet, flush tank, private use	Fixture schedule
Combined WSFU for that fixture	2.2	Appendix E fixture-unit table
Two of them on one branch	4.4 WSFU	2 × 2.2 — add the other fixtures on the branch the same way

Worked example value only. Use the combined column for cold-plus-hot piping and the separate columns for hot-only or cold-only sections.

Common mistakes

Mistake	What to do instead
Adding fixture gpm straight across ("everything open at once")	Convert total WSFU to demand — fixture units account for the odds of fixtures running together
Forgetting the backflow preventer loss	Get the loss at design flow from the assembly’s flow curve; it often decides the pipe size
Sizing to the gauge reading on a Sunday morning	Use the utility’s minimum static pressure
Picking the size by friction only	Check velocity too — undersized copper at high velocity erodes and bangs
Leaving high pressure unregulated	Where static pressure exceeds the 604.8 limit, install a pressure-reducing valve
Assuming older fixture flows	Table 604.4 caps flow rates, and modern low-flow fixtures lower real demand — the UPC’s Water Demand Calculator exists for that reason

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Sizing the water service without guessing

Read Section 604 and Appendix E free on codes.iccsafe.org. ICC offers free read-only access to the 2021 and 2024 International Plumbing Code. This page explains the method; it does not reproduce the tables.

Water pipe sizing is a pressure budget. You start with what the utility guarantees at the main, spend some of it lifting water to the top floor, spend more on the meter and backflow preventer, and hold back what the worst fixture needs to work (Table 604.3). Whatever is left is the pressure you can afford to lose to pipe friction — and that decides the pipe size.

Demand comes from fixture units, not from adding gpm. Each fixture carries a water supply fixture unit (WSFU) value. Total them per pipe section and convert the total to a design flow; the conversion bakes in the odds that fixtures run at the same time. Table 604.4 caps how much each fixture may flow, which is why buildings full of low-flow fixtures use less water than the old demand curves predicted.

Worked example. With 60 psi at the main, the top fixture 25 ft up (10.8 psi), 6 psi through the meter, 12 psi through the backflow preventer, and 15 psi held back for a flushometer, the friction budget is 16.2 psi. Over 120 ft of pipe plus a 50% fitting allowance (180 ft), that is about 9 psi per 100 ft — pick the smallest pipe that stays under it at design flow without exceeding your velocity limit.

Western jurisdictions on the Uniform Plumbing Code (IAPMO) size water piping under UPC Chapter 6 and Appendix A, and many allow the Water Demand Calculator in UPC Appendix M for dwellings. The idea is the same, but the fixture-unit values and demand curves differ — do not mix the two codes.

For tube dimensions and wall types see the copper tube size chart and the steel pipe schedule chart. The drain side is covered in drainage fixture units and pipe sizing, and support spacing in pipe hanger spacing. Field PM keeps submittals, pressure-test records, and inspection photos with the job.

FAQ

What is a water supply fixture unit (WSFU)?+

A WSFU is a load factor that rates how much a fixture contributes to peak demand, based on its flow, how long it runs, and how often it is used. You total the WSFU on each pipe section and convert the total to a design flow in gpm; the conversion accounts for the fact that not every fixture runs at once. The values are in IPC Appendix E, readable free at codes.iccsafe.org.

What is the difference between IPC Table 604.3 and Table 604.4?+

Table 604.3 is the minimum — the flow and flowing pressure each fixture needs to work. Table 604.4 is the maximum — the most water a faucet, showerhead, or flushing fixture is allowed to use. You design so every fixture gets at least its 604.3 pressure while the fixtures themselves meet the 604.4 limits.

How do I account for elevation in water pipe sizing?+

A column of water loses 0.433 psi of pressure for every foot of rise. Multiply the height of the highest fixture above the main by 0.433 and subtract it from the available pressure before you work out the friction budget.

When is a pressure-reducing valve required?+

IPC 604.8 requires an approved pressure-reducing valve where the static pressure inside the building would exceed its limit. Check your edition for the exact threshold and the strainer and bypass requirements that go with it.

How does the UPC size water piping?+

Jurisdictions on the Uniform Plumbing Code (IAPMO, common in the western US) size water piping under UPC Chapter 6 and Appendix A, with their own fixture-unit values. For single- and multi-family dwellings, UPC Appendix M allows the IAPMO Water Demand Calculator, a statistical method built for low-flow fixtures. Consult the UPC edition your jurisdiction adopted.

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