Pipe Size Calculator

Size the water supply main for a house by the fixture unit method: count bathrooms and appliances for WSFU, convert to peak gpm, and pick a pipe for the pressure and run.

By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.

Quick answer: Add up the water supply fixture units for every fixture, then pick the smallest building supply that serves that total at your pressure and developed length under UPC Table 610.4. Two full baths, a half bath, kitchen sink, dishwasher, clothes washer and two hose bibbs total 25.5 WSFU, about 17.3 gpm at peak, which calls for a 1 in main at 46 to 60 psi over 61 to 100 ft.

Results

Total fixture units: 25.5 WSFU; Full baths 12, half baths 3, kitchen 1.5, dishwasher 1.5, washer 4, hose bibbs 3.5
Estimated peak demand: 17.3 gpm; Hunter curve estimate for flush-tank fixtures; fixtures rarely all run at once
Recommended main supply size: 1 in; 1 in serves up to 33 WSFU at 46 to 60 psi with a 61 to 100 ft main; 3/4 in stops at 17
Branch guidance: 1/2 in up to 4 WSFU, 3/4 in up to 12; At 46 to 60 psi. A full bath group is 6 WSFU, so feed each bathroom with 3/4 in and drop to 1/2 in at the individual fixtures
Planning estimate only: UPC Table 610.4 method; Meter size, elevation, water heater, softener and filter losses, and local amendments change the answer. The plumbing code and a licensed plumber govern.

Pipe Size formula

Total WSFU = Full baths x 6 + Half baths x 3 + Kitchen sinks x 1.5 + Dishwashers x 1.5 + Clothes washers x 4 + 2.5 for the first hose bibb + 1 for each additional
Peak gpm (Hunter curve): 10 WSFU = 8, 20 = 14, 30 = 20, 40 = 24, 60 = 32, 100 = 43, interpolated between points
Main size = smallest pipe whose WSFU limit at the pressure and developed length is at or above the total
Limits used at 46 to 60 psi, 61 to 100 ft: 3/4 in 17, 1 in 33, 1-1/4 in 67, 1-1/2 in 151 WSFU (up to 60 ft: 20, 39, 78, 151)

Worked example

A house with three full baths, one half bath, a kitchen sink, dishwasher, clothes washer and three hose bibbs, on a well system at 30 to 45 psi with a 180 ft main. Fixture units: 3 x 6 = 18, plus 3 for the half bath, 1.5 + 1.5 + 4 for the kitchen and laundry, and 2.5 + 1 + 1 = 4.5 for the hose bibbs, for 32.5 WSFU. On the Hunter curve that is between 30 WSFU (20 gpm) and 40 WSFU (24 gpm): 20 + 2.5 / 10 x 4 = 21 gpm. At 30 to 45 psi over 151 to 200 ft the limits are 6 WSFU for 3/4 in, 15 for 1 in, 28 for 1-1/4 in and 57 for 1-1/2 in, so the main has to be 1-1/2 in.

Move the same house to city water at 46 to 60 psi with the meter 50 ft from the house and the limits become 20, 39, 78 and 151 WSFU. Now 32.5 WSFU fits a 1 in main, two sizes smaller. Pressure and run length matter as much as the fixture count.

Quick reference

Maximum WSFU by building supply size and developed length (UPC Table 610.4 basis)
Pressure	Supply size	Up to 60 ft	61 to 100 ft	101 to 150 ft	151 to 200 ft
30 to 45 psi	3/4 in	16	12	9	6
30 to 45 psi	1 in	25	21	17	15
30 to 45 psi	1-1/4 in	47	38	32	28
30 to 45 psi	1-1/2 in	124	91	70	57
46 to 60 psi	3/4 in	20	17	14	11
46 to 60 psi	1 in	39	33	28	23
46 to 60 psi	1-1/4 in	78	67	52	44
46 to 60 psi	1-1/2 in	151	151	128	105

How the fixture unit method works

Plumbing codes size supply piping by probability rather than by adding up faucet flows. Each fixture gets a weight in water supply fixture units that reflects its flow and how often it runs; a bathroom group counts 6, a washer 4, a kitchen sink 1.5. The total goes into a table (UPC Table 610.4) that lists the largest total each pipe size can serve at a given pressure range and developed length from the meter to the farthest fixture. Lower pressure or a longer run means more friction loss, so the same fixture count needs a bigger pipe.

This calculator uses the UPC method with the typical meter for each supply size: a 3/4 in meter for 3/4 and 1 in supplies, a 1 in meter for 1-1/4 in and a 1-1/2 in meter for 1-1/2 in. The Hunter curve figure is an estimate of the peak flow that total is likely to produce, useful for checking a tankless water heater, softener or meter rating.

What changes the answer

Static pressure is the starting point: measure it at a hose bibb with a gauge, then subtract 0.433 psi for every foot the highest fixture sits above the meter, plus the losses in the meter, backflow preventer, softener and water heater. A second-floor shower 20 ft above a 50 psi meter with a softener in the line can be down to 35 psi before the piping adds friction, which drops the house into the lower pressure band. Developed length is the actual pipe route, not the straight-line distance.

PEX has a smaller bore than copper of the same nominal size, so many plumbers run PEX one size larger for the main, and a manifold system with dedicated 1/2 in home runs changes the branch picture entirely. Local codes may use the IPC method, apply different fixture unit values or require a minimum 1 in service. Treat this as a planning number and have a licensed plumber size the system under the code adopted where you live.

Key facts

IPC Appendix E assigns 6 water supply fixture units to a private bathroom group with a flush-tank toilet, lavatory and tub or shower; the UPC lists the fixtures separately (2.5 + 1 + 4 = 7.5). A clothes washer is 4 and a kitchen sink or dishwasher 1.5.
Fixture units are not gallons per minute. Hunter's curve, published in 1940 and still the basis of plumbing codes, converts them to a probable peak flow: 20 WSFU is about 14 gpm and 100 WSFU only about 43 gpm.
Note 2 of UPC Table 610.4 requires a building supply of at least 3/4 in nominal size, no matter how few fixtures a house has.
A standard 5/8 x 3/4 in residential water meter is rated by AWWA for 20 gpm continuous; a 3/4 in meter for 30 gpm and a 1 in meter for 50 gpm.
The UPC caps static pressure at 80 psi (a pressure-reducing valve is required above that) and requires at least 15 psi at the highest fixture during peak demand.

Frequently asked questions

What size water line do I need for a 3 bathroom house?

Three full baths (18 WSFU) with a half bath, kitchen sink, dishwasher, washer and two hose bibbs total 31.5 WSFU. At 46 to 60 psi with a main up to 100 ft that is a 1 in supply, which serves up to 33 WSFU; at 30 to 45 psi or a longer run it becomes 1-1/4 in.

Is a 3/4 inch water line enough for a house?

For a small house, yes. At 46 to 60 psi with the meter within 100 ft, 3/4 in serves up to 17 WSFU: one full bath, a kitchen sink, dishwasher, washer and a hose bibb come to 15.5. Add a second bathroom and you are at 21.5, which needs 1 in.

How many gpm does a house use?

The Hunter curve converts 25.5 WSFU, a typical two and a half bath house, to about 17 gpm of probable peak demand. That is the number to compare with a meter rating, a well pump or a tankless water heater, not the sum of every faucet.

Does PEX need to be a size larger than copper?

Often. 3/4 in PEX has a 0.671 in bore against 0.785 in for type L copper, about 27% less area, so a long 3/4 in PEX main carries noticeably less. Many plumbers run 1 in PEX where 3/4 in copper would have been used.

Sources and references

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