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 11, 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.

Your measurements

A column or tube: volume is the circular area (from the diameter) times the height.

Full bathroomsToilet, lavatory and tub or shower: 6 WSFU each for a flush-tank bath group (IPC Appendix E; the UPC counts the fixtures separately at about 7.5).Half bathroomsToilet and lavatory: 3 WSFU each.Kitchen sinks1.5 WSFU each.Dishwashers1.5 WSFU each.Clothes washers4 WSFU each.Hose bibbs2.5 WSFU for the first, 1 for each additional (UPC Table 610.3).Water pressure at the meterRead it with a gauge on a hose bibb with nothing running. If there is a PRV, use its outlet setting, usually 50 to 60 psi, not the street pressure. On a well use the pressure switch cut-in, typically 40 psi.Developed length of the mainFollow the pipe route from the meter to the farthest fixture, including the drops and risers; a straight-line guess reads short and picks a pipe one size small.

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Total fixture units25.5 WSFUAll results

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 gpm is the number to hold against the meter, the softener and a tankless heater. AWWA C700 rates a 5/8 x 3/4 in meter at 20 gpm maximum, 3/4 in at 30 and 1 in at 50. A softener has a rated service flow on its sheet, and a tankless heater wants a 3/4 in feed and pulls 5 to 8 gpm at full fire.

What changes the answer

Static pressure is the starting point: read it with a gauge on a hose bibb with nothing running. If there is a PRV, use its outlet setting, not the street pressure. A PRV, required where static exceeds 80 psi (UPC 608.2, IPC 604.8), is usually set to 50 to 60 psi and loses another 10 to 15 psi at peak flow. Then subtract 0.433 psi for every foot the highest fixture sits above the meter. Subtract the equipment too. The meter takes 3 to 5 psi at rated flow and a dual-check backflow preventer 5 to 10. A softener takes 10 to 15 psi at 10 gpm and a loaded cartridge filter 5 to 10. A second-floor shower 20 ft above a 55 psi PRV with a softener in the line is under 30 psi before the piping adds friction. That drops the house a full pressure band. Developed length is the pipe route, not the straight-line distance.

PEX has a smaller bore than copper: 3/4 in PEX is 0.671 in inside against 0.785 in for type L copper. Each crimp or clamp insert fitting chokes it to about 0.5 in, so pros run 1 in PEX for the main and trunks, or use ASTM F1960 expansion fittings, which keep most of the bore. A manifold with dedicated 1/2 in home runs changes the branch picture, since each run carries one fixture. Galvanized steel from before the 1960s scales shut. A 3/4 in galvanized main with 50 years of rust passes less than 1/2 in copper, and replacing it fixes more low-pressure complaints than upsizing ever does. The IPC method (Appendix E) and UPC Table 610.4 land within one size of each other. Some utilities require a 1 in service and set tap fees by meter size, so ask before you upsize the meter.

What this calculator leaves out

The meter and its tap or capacity fee. The service line from the meter: type K copper, 200 psi CTS polyethylene or 1 in PEX, buried below the frost line and at least 12 in deep. The main shutoff, the PRV and the thermal expansion tank a PRV requires (UPC 608.3, IPC 607.3), and the backflow preventer. Sizing of the hot side and each fixture branch, the water heater, and the permit with its pressure test. It also assumes the sewer is in its own trench; a water service sharing the trench sits on a shelf 12 in above the sewer (IPC 603.2, UPC 720.1).

Code limits that apply

UPC Table 610.4checked
Main serves no more WSFU than the table allows at its pressure band and developed length
If you miss it: Two showers and the washer together starve the top floor, and the fix is a new main trenched back to the meter
UPC 610.8
Building supply no smaller than 3/4 in nominal, whatever the fixture count
If you miss it: A 1/2 in service to a small house is red-tagged at the rough-in and the trench reopened to run 3/4 in
UPC 608.2, IPC 604.8
Pressure-reducing valve where static pressure exceeds 80 psi (5.5 bar)
If you miss it: Washer hoses, supply lines and fill valves at 90 psi hammer and burst, and the plumbing final waits on a PRV
UPC 608.3, IPC 607.3
Thermal expansion tank on a closed system with a PRV, check valve or backflow preventer
If you miss it: The water heater T&P valve drips on every heating cycle and the relief line weeps into the pan
IPC 305.4
Water service buried at least 12 in deep and 6 in below the frost line
If you miss it: A shallow service freezes solid in a cold snap and splits, leaving the house dry until the ground thaws
IPC 603.2, UPC 720.1
Water service in a sewer trench sits on a shelf 12 in above the sewer, or 5 ft away
If you miss it: A water line laid beside the sewer fails the underground inspection and is dug back out before backfill

Provision	Limit	If you miss it
UPC Table 610.4checked	Main serves no more WSFU than the table allows at its pressure band and developed length	Two showers and the washer together starve the top floor, and the fix is a new main trenched back to the meter
UPC 610.8	Building supply no smaller than 3/4 in nominal, whatever the fixture count	A 1/2 in service to a small house is red-tagged at the rough-in and the trench reopened to run 3/4 in
UPC 608.2, IPC 604.8	Pressure-reducing valve where static pressure exceeds 80 psi (5.5 bar)	Washer hoses, supply lines and fill valves at 90 psi hammer and burst, and the plumbing final waits on a PRV
UPC 608.3, IPC 607.3	Thermal expansion tank on a closed system with a PRV, check valve or backflow preventer	The water heater T&P valve drips on every heating cycle and the relief line weeps into the pan
IPC 305.4	Water service buried at least 12 in deep and 6 in below the frost line	A shallow service freezes solid in a cold snap and splits, leaving the house dry until the ground thaws
IPC 603.2, UPC 720.1	Water service in a sewer trench sits on a shelf 12 in above the sewer, or 5 ft away	A water line laid beside the sewer fails the underground inspection and is dug back out before backfill

Rows marked checked produce a warning in the results when an entry crosses the limit. The others are on the plans or on site and this calculator does not test them. Local amendments can be stricter; the adopted edition and the inspector govern.

Mistakes that cost money

Sizing from street pressure with a PRV in line: a 55 psi PRV drops another 10 to 15 psi at peak flow, one band lower.
Ignoring softener and filter losses: a softener drops 10 to 15 psi at 10 gpm, a loaded cartridge filter 5 to 10; upsize the main.
A 3/4 in PEX main on crimp fittings: each insert fitting chokes the bore to about 0.5 in; run 1 in PEX or F1960 fittings.
Upsizing the meter without calling the utility: a 5/8 to 1 in meter swap brings a new tap and capacity fee, typically $1,000 to $5,000.
A PRV or check valve with no thermal expansion tank: the water heater T&P valve drips every heating cycle (UPC 608.3, IPC 607.3).
Water service laid in the sewer trench: it goes 12 in above the sewer on a shelf or 5 ft away (IPC 603.2, UPC 720.1).

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.
AWWA C700 rates a standard 5/8 x 3/4 in residential meter for 20 gpm maximum and 10 gpm continuous, a 3/4 in meter for 30 and 15, and a 1 in meter for 50 and 25.
The UPC caps static pressure at 80 psi (a pressure-reducing valve is required above that, UPC 608.2 and IPC 604.8) 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.

My shower drops when the washer runs. Do I need a bigger main?

Not always. Put a gauge on a hose bibb: under 50 psi with a PRV, turn the PRV up, to 80 psi at most (UPC 608.2). Bypass the softener and test again; a loaded softener or cartridge filter drops 10 to 15 psi. Galvanized steel from before the 1960s scales shut, and replacing it with 1 in PEX or 3/4 in copper ends the complaint. If the main is 3/4 in and the house counts over 17 WSFU at 46 to 60 psi, that is the point to upsize, and the meter with it.

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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