Water Supply Fixture Unit Calculator — WSFU to GPM
Every water supply pipe is sized from one number: the probable peak demand in gallons per minute. You get there in two steps — weight each fixture in water supply fixture units, then read the demand off Hunter's curve, which accounts for the fact that the fixtures are never all running at once. Count your fixtures below and both numbers come out.
Count your fixtures
Decides which column of the demand curve is read. Valve systems demand far more for the same fixture count.
Residential fixtures
Commercial fixtures
Probable peak demand
Flush-tank system — tanks refill slowly, so peak demand is far below the sum of the fixture flows.
Total WSFU
16.4
Cold
12.4
Hot
6.4
See the breakdown
Planning estimate. The fixture-unit sizing method lives in IPC Appendix E, which is only enforceable where your jurisdiction has adopted it. A licensed plumber and your AHJ have final say.
About these numbers
The fixture weights and the demand curve here are the commonly published water supply fixture unit values and Hunter's-curve demand figures. They approximate IPC Appendix E Tables E103.3(2) and E103.3(3), and where your jurisdiction has adopted Appendix E, the code tables are the authority — check a result against the book before it goes on a permit drawing. Hunter's original method is US government work from 1940 (NBS report BMS65) and is not itself proprietary; the code tables' exact values are. The error runs toward larger pipe, for the low-flow reason in the FAQ below.
The formula, explained in plain English
Two steps. The first is arithmetic; the second is statistics, and it is the one that surprises people.
What a fixture unit actually is
A dimensionless weight, not a flow rate. It bundles how much water a fixture uses, how long it runs, and how often — so one number can compare a hose bibb with a dishwasher.
Why cold + hot ≠ total
A lavatory is 0.5 cold, 0.5 hot, 0.7 total. You never draw full hot and full cold in the same instant, so the code assigns the combined weight separately rather than adding the two.
Bathroom groups are a shortcut
A group counts as 3.6 WSFU rather than the 4.3 you would get adding a water closet, lavatory and tub separately — because fixtures in one room compete for one person.
Where the curve flattens
Past roughly 1,000 fixture units the tank and valve columns converge — at that scale the averaging has done all the work it can, and demand grows almost linearly again.
Worked examples
A house, a small office, and the case that catches people out — the same fixture count on flushometer valves.
Two-bathroom house — the defaults
2 bathroom groups on flush tanks, kitchen sink, dishwasher, clothes washer, 2 hose bibbs.
1 × kitchen sink (1.4) = 1.4
1 × dishwasher (1.4) = 1.4
1 × clothes washer (1.4) = 1.4
2 × hose bibb (2.5) = 5.0
total = 16.4 WSFU → demand 18.2 gpm
Result: 18.2 gpm. The connected fixture capacity is about 30 gpm, so the curve has cut the design flow by roughly 40% — and that is before any pipe is chosen.
The same house, flushometer valves
Change nothing but the supply control on the water closets.
→ demand 32.1 gpm instead of 18.2
ratio = 1.77×
Result: 32.1 gpm — a 77% jump from one dropdown. On the pipe-size calculator's defaults, 18 gpm rides a 1-inch copper main and 32 gpm needs 1-1/2 inch — two sizes up from one dropdown. This is why the supply control matters more than almost any other input.
Small office — 50 fixture units
A build-up of public water closets, lavatories and a service sink reaching 50 WSFU on tanks.
= 0.58 gpm per fixture unit
versus the house at 16.4 WSFU → 1.11 gpm per fixture unit
Result: triple the fixture units, but only 60% more flow. Demand per fixture unit almost halved — which is why a large building's main is never proportional to its fixture count.
Fixture units to gpm — the demand curve
Rendered from the same data the calculator uses, so the two cannot disagree. Watch the last column: demand per fixture unit falls by more than 5× across the table. That single fact is the difference between a properly sized main and a wildly oversized one.
| Fixture units | Flush tanks | Flush valves | gpm per unit (tanks) |
|---|---|---|---|
| 5 | 9.4 gpm | 15.0 gpm | 1.88 |
| 10 | 14.6 gpm | 27.0 gpm | 1.46 |
| 20 | 19.6 gpm | 35.0 gpm | 0.98 |
| 30 | 23.3 gpm | 41.0 gpm | 0.78 |
| 50 | 29.1 gpm | 51.5 gpm | 0.58 |
| 100 | 43.5 gpm | 67.5 gpm | 0.44 |
| 200 | 65.0 gpm | 91.5 gpm | 0.33 |
| 500 | 124.0 gpm | 142.0 gpm | 0.25 |
Sources & standards: IPC 2021 Appendix E — Sizing of Water Piping System, Tables E103.3(2) and E103.3(3); IPC 604.3 maximum fixture flow rates; Roy Hunter, Methods of Estimating Loads in Plumbing Systems, NBS Building Materials and Structures Report BMS65 (1940); ASPE design guidance. Appendix E applies only where adopted. Roughly fifteen states use the UPC or a derivative, whose fixture-unit values differ. Local amendments override the model code.
Frequently asked questions
Common questions about water supply fixture units, Hunter's curve, and peak demand.
How many fixture units is a typical house?
A two-bathroom house with a kitchen sink, dishwasher, clothes washer and two hose bibbs comes to about 16.4 WSFU, which is roughly 18 gpm of probable peak demand on a flush-tank system. Those are this calculator's defaults. A three-bathroom house lands nearer 22 WSFU and 20 gpm — the demand curve flattens fast, which is the whole point of it.
Why isn't peak demand just the sum of the fixture flow rates?
Because the fixtures are never all running at once. Those defaults add up to about 30 gpm of connected fixture capacity but only 18 gpm of probable demand. Roy Hunter's 1940 work at the National Bureau of Standards turned that into a probability curve: the more fixtures you add, the smaller the fraction that runs simultaneously. At 5 fixture units you get 1.88 gpm per unit; at 200 units you get 0.33.
What is the difference between a flush tank and a flushometer valve?
A flush tank refills slowly through a small fill valve, so it barely dents the system. A flushometer valve takes its whole flush straight off the supply in a few seconds — a huge, brief draw. The same 16.4 fixture units demand about 18 gpm on tanks and 32 gpm on valves, so guessing wrong here can undersize a main by two pipe sizes. Valve systems are almost always commercial.
Are WSFU and DFU the same thing?
No, and mixing them up is the most common fixture-unit mistake. WSFU weights a fixture by how hard it pulls on the supply; DFU weights it by what it puts down the drain. The same lavatory is 0.7 WSFU and 1 DFU. Use the Drainage Fixture Unit Calculator for the gravity side.
Does the cold and hot split have to add up to the total?
No — and it deliberately doesn't. A lavatory is 0.5 cold, 0.5 hot, and 0.7 total, not 1.0. The code assigns the total separately because you never draw full hot and full cold at the same instant. Size the cold main on the total, and size the hot distribution on the hot column.
Is this method actually in the code?
The fixture-unit sizing method lives in IPC Appendix E, and an appendix is only enforceable where the jurisdiction has specifically adopted it. Many have not, in which case supply sizing falls to engineering judgement or a local method. Check before you rely on it for a permit.
Does the curve still work with modern low-flow fixtures?
It runs high. Hunter calibrated the curve on fixtures that used several times the water today's do, and IPC 604.3 has cut fixture flow rates hard since — a water closet went from 5 gallons a flush to 1.6. The result is that a WSFU-derived demand overstates what a modern building actually draws, so the error pushes you toward larger pipe. Conservative, but worth knowing when a result looks generous.
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