Type the water supply fixture-unit total from the code your jurisdiction enforces. Get probable demand and the minimum pipe bore.
600 FU → 137 gpmHunter’s worked example: a 77% diversity discount100 flush-tank bathrooms at six fixture units each. US National Bureau of Standards (now NIST), BMS65 (1940), public domain, retrieved September 5, 2026.
Water supply fixture units convert to probable demand through Hunter’s curves, and demand converts to a minimum pipe bore by Q = 2.448 × d² × v. 600 fixture units on flush tanks is 137 gpm, a 2.64 inch bore at 8 ft/s: 3 inch Type L copper.
Take the fixture-unit values from your adopted code.
Work out demand and bore
Arithmetic on your total. Nothing is stored or emailed.
A screening check, not a design or a permit document. No fixture-unit table and no pressure calculation.
Worked examples
Three loads, run through the calculator
Hunter’s own example
600 fixture units on flush tanks: 137 gpm, a 2.64 in bore at 8 ft/s, so 3 in Type L copper.
A small building
60 fixture units on flush tanks: 24 gpm, a 1.11 in bore, so 1¼ in Type L copper.
Add a continuous load
600 fixture units on flush valves plus 20 gpm of irrigation: 154 + 20 = 174 gpm, so 3½ in.
HyrePlumber calculation from the engine on this page. A hydraulic bore, not a code pipe size.
First: which plumbing code does your jurisdiction enforce?
The US has two model plumbing codes, and they give different fixture-unit values to the same fixture.
The two codes. The International Plumbing Code (IPC) is published by the International Code Council. The Uniform Plumbing Code (UPC) is published by IAPMO. States adopt one, and states and cities amend them.
Why there is no table here. Both codes are copyrighted, and we could not read either table at its source to check it. A table we cannot check could undersize a supply main.
One call settles it. Ask your building department: which code, which edition, and where are the local amendments published? Then sum the fixture units from that table and type the total.
What to ask, and what each answer changes
Ask
Why it matters
What changes
Which code and edition?
IPC and UPC values differ, and editions change
The fixture-unit total, and every size after it
Any local amendments?
A city can amend the table you are reading
The state answer may not be the rule
Supply (WSFU) or drainage (DFU)?
Two separate systems with different values
Everything: this tool refuses drainage
What pressure and pipe length?
Flow is half the sizing; pressure is the other half
A pipe big enough for flow can still fail on pressure
HyrePlumber’s questions, not a code requirement.
A fixture unit is a load weight, not gallons per minute
Hunter, 1940: “Fixture unit, or load factor, is a numerical factor which measures on some arbitrary scale the load-producing effect of a single plumbing fixture of a given kind. The use of the fixture unit makes it possible to reduce the load-producing characteristics to a common basis.”
Not a flow. “This unit in the weight scale is not a definite unit of flow but is simply a load factor to be applied through a relation between m and n.”
The weights. “On the basis of average values, the relative weights of the three fixtures become 10 for flush valves for water closets, 4 for bathtubs (total hot- and cold-water supply), and 5 for flush tanks.” They compare load, not gallons.
Adding flows: “Adding up numbers like these to cover all the fixtures in an entire building distribution system would give the total demand for water usage in gpm, if all of the fixtures were operating and flowing at the same time — which of course does not happen.”
What one fixture draws while it runs
These are flows for one fixture on its own. They are not fixture units, and adding them does not size a pipe.
Fixture
Flow while running
Drinking fountain
0.75 gpm
Lavatory faucet
2.00 gpm
Lavatory faucet, self-closing
2.50 gpm
Sink faucet, water-closet tank ball cock
3.00 gpm
Bathtub faucet, shower head, laundry tub faucet
4.00 gpm
Sill cock, hose bibb, wall hydrant
5.00 gpm
Flush valve (depending on design)
3.50 gpm
Copper Development Association, Copper Tube Handbook (2026 print edition), retrieved September 5, 2026.
More fixtures, less flow per fixture
Doubling the fixture units does not double the demand. That flattening is the method.
The discount. On flush valves, 80 fixture units estimate 64 gpm (0.80 per unit); 800 estimate 182 gpm (0.23 per unit). Ten times the load, under three times the flow.
Why. Each fixture runs for a short time between long gaps, so only some run at once. Hunter designed for a high percentile of that, not for every tap open together.
Private bathrooms. “It is suggested that a bathroom group in a residence or apartment, or a private bathroom in a hotel be given one-half the total weight for the same fixtures in public or congested service in estimating supply demands.”
Hunter’s printed demand figures
Probable demand against total fixture units, from BMS65 Table 9. Demand per unit falls as the load grows.
Fixture units
Flush valves
Flush tanks
20
36 gpm
12 gpm
80
64 gpm
30 gpm
400
125 gpm
96 gpm
1,500
269 gpm
310 gpm
US National Bureau of Standards (now NIST), BMS65 Table 9, converted from fixture counts to fixture units with Hunter’s Table 7 weights (10 per flush valve, 5 per flush tank). Public domain. The two curves cross between about 880 and 1,040 fixture units.
The sizing method, step by step
This tool does steps 3 and 4: the two that need no copyrighted table and no site visit.
1
Find the governing code and edition
IPC or UPC, the edition year and the local amendments, from the building department.
2
Sum the fixture units from that table
For the part you are checking: the service, a riser or one branch. Hot-only and cold-only branches carry their own values.
3
Turn fixture units into probable demand
Through the curve, then add continuous demand in full. Hunter: “This estimate, of course, applies only to the main supply or service pipe for the building and does not include any continuous demand, such as that for lawn sprinklers or air conditioning, etc.”
4
Check the bore against a velocity limit
Q = 2.448 × d² × v: Q in gpm, d the inside diameter in inches, v in feet per second.
5
Then check pressure, which this tool does not
Supply pressure, minus meter loss, 0.434 psi per foot of height and friction over the pipe length, must leave enough at the highest fixture.
The curve was set on a six-gallon toilet
Hunter calibrated his curves on fixtures that used far more water than today’s. The error runs one way: toward larger pipe.
about 6 galper flush in older toiletsEPA WaterSense
1.6 galfederal maximum per flush todayEPA WaterSense
1.28 galor less for a WaterSense toiletEPA WaterSense
1940year Hunter published the curvesNBS BMS65
Hunter’s own test for the method: “The proof of the adequacy of the proposed method of estimating the demand loads to be expected in building-water-supply systems will, in the end, depend on its success in actual trial over a period of years.” The comparison with today’s fixtures is HyrePlumber’s, not EPA’s.
Supply and drainage fixture units are different systems
Carrying a number from one to the other gives a plausible, wrong answer. The calculator refuses drainage.
Supply (WSFU)
Drainage (DFU)
What it loads
Pressurized supply pipe
Gravity drain pipe, stacks and the sewer
What sets the value
Flow drawn and how frequently it is drawn
Discharge into the drain. Hunter: “The probability function has only a limited application to the drainage systems of buildings.”
Does flush type matter?
Yes: a flush valve draws harder than a tank
Largely no. “The principal exception is the water closet, which for the drainage system will in general have the same weight for a given congestion of service whether the water-supply control is by flush valve or flush tank.”
What turns it into a size
A demand curve, then velocity and pressure
A code table keyed to slope, stack or branch
What else governs
Height, pipe length, meter and fittings
Venting
Hunter’s two sentences are quoted from BMS65; the rest is HyrePlumber’s summary.
One input changed at a time
Every figure comes from running this page’s engine. The baseline is Hunter’s own example: 600 fixture units on flush tanks.
Change
Probable demand
Minimum bore
Smallest Type L copper
Baseline: 600 FU, flush tank, 8 ft/s
137.0 gpm
2.64 in
3 in (2.95 in bore)
Flush valves instead
154.0 gpm
2.80 in
3 in (2.95 in bore)
Hot water at 5 ft/s
137.0 gpm
3.35 in
3½ in (3.43 in bore)
Plus 20 gpm continuous
157.0 gpm
2.83 in
3 in (2.95 in bore)
Double the load: 1,200 FU
253.6 gpm
3.60 in
4 in (3.91 in bore)
A small building: 60 FU
24.0 gpm
1.11 in
1¼ in (1.26 in bore)
30 FU
15.0 gpm
0.88 in
1 in (1.02 in bore)
Changing flush type moves demand 12% without changing the size; changing the velocity limit moves no demand and does change the size. A hydraulic bore, not a code pipe size.
The velocity limit the tool applies
Copper Development Association: “To avoid excessive system noise and the possibility of erosion-corrosion, the designer should not exceed flow velocities of 8 feet per second for cold water and 5 feet per second in hot water up to approximately 140°F.”
Hotter water: “In systems where water temperatures routinely exceed 140°F, lower flow velocities such as 2 to 3 feet per second should not be exceeded.”
Small tube. “In addition, where 1/2-inch and smaller tube sizes are used, to guard against localized high velocity turbulence due to possibly faulty workmanship (e.g. burrs at tube ends which were not properly reamed/deburred) or unusually numerous, abrupt changes in flow direction, lower velocities should be considered.”
Type L copper. Types K, L and M share an outside diameter and differ in wall, so their bores differ. The tool reports Type L. PEX and CPVC differ again, and PEX fittings can narrow the bore at each joint.
A floor, not an answer. A code table or the pressure check can require a larger pipe. Nothing here justifies going smaller than the drawing.
How fixture-unit sizing goes wrong
Adding fixture flows to get a design flow
That sizes for every tap open at once, the case the method exists to avoid.
Using drainage values for supply, or the reverse
Same fixture, different number, different physics.
Quoting fixture units without naming the code
IPC, UPC, editions and amendments differ. An unnamed value cannot be checked.
Running continuous demand through the curve
Irrigation or cooling that runs steadily is added in full, not discounted.
Stopping at flow and skipping pressure
A pipe big enough for the flow can still leave too little pressure upstairs.
Size each branch, not only the service
The method applies segment by segment, each with its own fixture-unit total.
Questions this calculator answers
What is a fixture unit?
A load weight on an arbitrary scale, not a flow rate. Roy B. Hunter defined it in 1940 so different fixtures could be added together. A total converts to gallons per minute only through a probability curve.
Why does the tool not list fixture-unit values?
The IPC and UPC give different values for the same fixture, states adopt one and amend it, and both are copyrighted. We could not check either table at its source, so you bring the total from your code.
IPC or UPC: which applies to me?
Ask your building department three things: which model code, which edition, and where the local amendments are published. The state answer is a starting point; cities can amend on top.
Why is a fixture unit not gallons per minute?
Because fixtures do not all run at once. Hunter’s curve returns the demand to design for. At 600 fixture units on flush tanks that is 0.23 gpm per unit.
Does the tool size drains?
No. Drainage fixture units are a separate system, and drain size depends on slope, stack or branch position and venting. Selecting drainage returns no size.
Is a 1940 curve still valid?
Both modern codes descend from it. It was calibrated on toilets using about 6 gallons a flush, against 1.6 today, so it errs toward larger pipe: the safe direction.
Can I size my own pipe with this?
No. It is a screening check on a plan or a bid: no fixture-unit table, no pressure calculation, no drains. Plumbing work is permitted work, sized against your adopted code.
Copper Tube Handbook (2026 print edition) (Copper Development Association, retrieved September 5, 2026). Velocity limits, per-fixture flows and Type L copper inside diameters (Table 14.2b).
Residential toilets (US Environmental Protection Agency, WaterSense, retrieved September 5, 2026). Toilet flush volumes then and now. The comparison with Hunter’s calibration is HyrePlumber’s.
International Plumbing Code (International Code Council, retrieved September 5, 2026). One of the two model codes. Its tables are not reproduced here.
Uniform Plumbing Code (International Association of Plumbing and Mechanical Officials, retrieved September 5, 2026). The other model code. Its tables are not reproduced here.
Related
Pipe materialsCopper types, PEX and CPVC, and how bores differ.