Fixture Units Calculator
Calculate total Water Supply Fixture Units (WSFU) and Drainage Fixture Units (DFU) for your project.
🎯 Where Fixture Units Came From
The fixture unit is one man's idea, and it is older than most of the plumbing it sizes. Roy B. Hunter published it for the National Bureau of Standards in 1940, in Methods of Estimating Loads in Plumbing Systems (Building Materials and Structures Report BMS65). His problem was that a building's fixtures are almost never all running at once, so adding up flow rates grossly oversizes the pipe. His answer: give each fixture a weight reflecting how much load it produces and how often it is used, total the weights, and read a probable peak demand off a curve derived from the probability function.
Hunter's own weights, from Table 7 of that report, still show through every modern code table — a public flush-valve water closet at 10 units, a flush-tank closet at 5, a public bathtub or shower head at 4, a public lavatory at 2, a private bathroom group at 8 with a flush valve or 6 with a flush tank.
Method Reference
The WSFU-to-GPM step reads off Hunter's curve, and there are two of them. Hunter did not publish one demand curve — he published a family, and he is explicit that you must pick: apply the weighted total “to a load curve for flush valves or a load curve for flush tanks, according to which type of supply is to be used.” This calculator interpolates the values he tabulated in Table 9 of BMS65, and chooses the curve for you: enter any flush-valve closet or urinal and it reads the flush-valve curve, otherwise the flush-tank curve. The difference is not small. At 80 fixture units his flush-valve curve gives 64 GPM and his flush-tank curve gives 30. A tank-flushed house and a valve-flushed office block can total the same fixture units and need supply pipe of quite different size.
Two limits worth knowing. Hunter's published points stop at 3,000 fixture units on the flush-valve curve and 1,500 on the flush-tank curve; past those the calculator continues at the slope of the last segment and says so, because that is extrapolation and not a reading. And a real system mixes fixture types, whereas each curve was derived for one. Hunter warns about exactly this: the design load for a system “should not be the sum of the design loads computed separately for each kind of fixture, even though the individual curves may be correct”, because simultaneous use across different fixture types is its own probability problem. Totalling the units and reading one curve is his own simplification, and it is what the codes adopted. Size from the demand table in the code your jurisdiction adopted before you buy pipe.
The suggested supply line is velocity-derived, not a table lookup. It is the smallest copper size that keeps the estimated peak flow at or below 8 ft/s. It takes no account of developed length, available pressure or pressure loss, all of which can force a larger size.
📊 Supply Units and Drainage Units Are Different Loads
WSFU measures demand on the pressurised supply side; DFU measures the load a fixture puts on gravity drainage. They are not interchangeable, and a fixture can be heavy on one and negligible on the other — a hose bibb draws hard on the supply and discharges nothing, a floor drain the reverse. Hunter noted the same asymmetry in 1940: a water closet carries the same drainage weight whether its supply is a flush valve or a flush tank, though its supply weights differ by a factor of two.
⚖️ The IPC and the UPC Do Not Agree
Which value applies to you is jurisdictional, and the two model families genuinely differ. The UPC's supply values are reproduced in full by the Minnesota Revisor of Statutes as part of the Minnesota Plumbing Code, free to read; the IPC's load values sit behind ICC's paywall, so nothing here is attributed to the IPC. Against the Minnesota text, several values in the table above are lower:
| Fixture (private) | This calculator | UPC as adopted in Minnesota |
|---|---|---|
| Clothes washer | 1.4 | 4.0 |
| Kitchen sink | 1.4 | 1.5 |
| Dishwasher | 1.4 | 1.5 |
| Laundry tray | 1.4 | 1.5 |
| Drinking fountain | 0.25 | 0.5 |
| Water closet, tank | 2.2 | 2.5 (1.6 GPF gravity tank) |
| Bathtub, shower, lavatory, bidet, hose bibb | identical in both | |
The clothes washer is the one that matters: at 1.4 against 4.0 it is nearly a third of the UPC load, and a laundry-heavy building totalled this way will come out light. The Minnesota text also declines to give flushometer-valve closets and urinals a fixture unit at all, sizing them by a separate section instead — where this calculator carries 6 and 5. If you work under a UPC jurisdiction, total your fixtures from the adopted table rather than from this one.
❓ Questions People Actually Ask
What is a water supply fixture unit?
A weighting factor, not a flow rate. Roy B. Hunter introduced it for the National Bureau of Standards in 1940 to express how much load a fixture puts on a supply system, accounting both for how much water it draws and how often it is used. Totalling the weights and reading a demand curve gives a probable peak flow far below the sum of the individual flow rates, because fixtures are rarely all in use at once. The values themselves are set by whichever plumbing code your jurisdiction adopted, and the IPC and UPC families do not assign the same numbers.
How to calculate water supply fixture unit?
Count each fixture type on the segment you are sizing, multiply by the value your adopted code assigns it, and add the products. Then convert that total to a probable peak flow using the demand table in the same code — the total is not itself a flow rate. Count per segment rather than per building, since a branch carries only what is downstream of it. Values differ between the IPC and UPC families, so a total means nothing without the code it came from, and the pipe size it implies still has to survive a pressure check.
How many fixture units does a 3/4 water line have?
There is no single number. Code sizing tables state fixture unit capacity as a function of available pressure, developed length and pipe material, so the same 3/4-inch line carries very different loads at 40 psi over 200 feet and at 80 psi over 40 feet. What is fixed is the hydraulic ceiling: 3/4-inch Type L copper has a 0.785-inch bore, so at the Copper Development Association's 8 ft/s cold-water limit it carries about 12 gallons per minute, and no fixture unit table can exceed that. Size from your adopted code's table, not from a rule of thumb.
A fixture unit total is an input to sizing, not a size. The peak flow shown here comes from an approximate curve, not from a code demand table, and the suggested supply line is a velocity check on copper that ignores length and pressure entirely.
Confirm every fixture unit value against the plumbing code adopted in your jurisdiction and its local amendments before you size anything from it. This does not replace a licensed plumber or an engineered design.
The origin of the fixture unit, the Table 7 weights, and the demand curves this calculator reads are from Roy B. Hunter's Methods of Estimating Loads in Plumbing Systems, National Bureau of Standards Report BMS65 (1940), published in full by NIST. The two demand curves are Hunter's Table 9, “Demand estimates in gallons per minute from figure 4”, converted from his fixture counts to fixture units with the Table 7 weights of 10 per flush-valve water closet and 5 per flush-tank water closet; the single-fixture end of each uses the 27 GPM and 4 GPM he assigns those fixtures in deriving figure 3. His instruction to apply the total “to a load curve for flush valves or a load curve for flush tanks, according to which type of supply is to be used”, and his warning against summing loads computed separately for each kind of fixture, are on pages 13 and 12 respectively. The UPC supply values are from Minnesota Rules 4714.0610, the Minnesota Plumbing Code's version of UPC Table 610.3 — Minnesota's adopted and amended text, so check your own state's. The 0.785-inch bore and 8 ft/s limit are from the Copper Development Association's Copper Tube Handbook. Once you have a peak flow, our pipe sizing calculator takes it from there. The questions above came from Google's People Also Ask panel for "water supply fixture units" on 31 July 2026.