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Water Pipe Size Calculator

Size water supply piping from fixture unit counts, pressure and developed length.

Fixtures served

Count every fixture downstream of the segment you are sizing. Fixture unit values are private-use figures; a fixture that runs continuously, such as an irrigation zone, is added to demand directly rather than as fixture units.

2.2 WSFU each

1.0 WSFU each

4.0 WSFU each

2.0 WSFU each

1.5 WSFU each

1.4 WSFU each

1.4 WSFU each

1.4 WSFU each

1.0 WSFU each

1.0 WSFU each

2.5 WSFU each

Pressure available
psi

Measure at a hose bibb with nothing running.

psi

A 3/4 in. meter at 20 gpm typically loses 5–8 psi. Add softeners and filters here.

psi

Most shower and tub valves want 15–20 psi flowing. Flushometers want 25.

ft

0.433 psi is lost per foot of rise, regardless of pipe size.

Piping

The standard wall thickness for potable water distribution in most jurisdictions.

ft

Measured run plus equivalent length for every fitting and valve. Adding 50% to the measured run is the usual rough allowance.

WSFU

Fixture units on the busiest branch. A full bath group is about 6 WSFU.

Hot recirculating lines and aggressive water call for the lower figure.

Minimum service and main

1"Copper, Type L

Carries 20.2 gpm at 7.9 ft/s and 20.3 psi of friction loss.
Total fixture units11 fixtures counted
21.7 WSFU
Probable demandHunter's curve, flush-tank system
20.2 gpm
Pressure available for friction18.78 psi per 100 ft over 180 ft
33.8 psi
Largest branch10.7 gpm at 7.1 ft/s
3/4"

Compliant, but running near the 8 ft/s limit

1" carries 20.2 gpm at 7.9 ft/s, consuming 20.3 psi of your 33.8 psi friction budget. Oversizing past this is not free: a larger main holds more water, so hot water takes longer to arrive at every fixture and you pay for the wasted volume on every draw.
Candidate sizes at 20.2 gpm — Copper, Type L
NominalInside dia.VelocityLoss per 100 ftLoss over runStatus
1/2"0.545 in27.8 ft/s243.42 psi438.2 psiVelocity too high
3/4"0.785 in13.4 ft/s41.24 psi74.2 psiVelocity too high
1" — minimum1.025 in7.9 ft/s11.26 psi20.3 psiCompliant
1-1/4"1.265 in5.2 ft/s4.05 psi7.3 psiCompliant
1-1/2"1.505 in3.7 ft/s1.74 psi3.1 psiCompliant
2"1.985 in2.1 ft/s0.45 psi0.8 psiCompliant
2-1/2"2.465 in1.4 ft/s0.16 psi0.3 psiCompliant
3"2.945 in1.0 ft/s0.07 psi0.1 psiCompliant
Pressure budget
ItemBasispsi
Static supply pressureMeasured with nothing running60.0
Elevation rise12 ft × 0.433 psi/ft−5.2
Meter, backflow and treatmentAs entered−6.0
Residual at the highest fixtureRequired flowing pressure−15.0
Available for pipe friction56% of supply33.8

What this assumes

  • Water supply fixture unit values follow the Uniform and International Plumbing Code fixture unit tables.
  • Velocity is held at or below 8 ft/s for copper and 5 ft/s where erosion is a concern.
  • Developed length includes an allowance for fittings expressed as equivalent length.

Important

Your jurisdiction adopts either the IPC or UPC, and the fixture unit values differ between them. Confirm which code applies before relying on a size for permitted work.

Understanding the result

Fixture units, not fixtures

Pipe is not sized by counting fixtures, because fixtures do not all run at once and do not all demand the same flow. Each fixture is assigned a water supply fixture unit value that reflects both its flow rate and how often it is used, and the total WSFU converts to a probable simultaneous demand in gallons per minute.

Pressure and length work against you

Available pressure is consumed by elevation (0.433 psi per foot of rise), by friction along the pipe, and by fittings. A long run to a second-floor bathroom can lose most of its static pressure, which is why the same fixture group can need a larger line in one house than another.

Why oversizing is not free

Oversized supply lines hold more water, so it takes longer for hot water to arrive at the fixture. That wastes water and energy every draw. Correct sizing — not maximum sizing — is what you are aiming for.

Branch versus trunk sizing

Fixture units accumulate as branches combine toward the meter. A bathroom group might total 8 WSFU, but the main line serving three bathrooms, a kitchen, and a laundry can exceed 40 WSFU. Size each segment from its own fixture unit total and available pressure at that point — not from the farthest fixture alone.

PEX manifold systems change the math

A home-run PEX manifold runs a dedicated line to each fixture, so each branch is sized for one fixture's demand rather than cumulative WSFU. That often allows 3/8 or 1/2 inch runs to individual fixtures while the trunk to the manifold stays at 3/4 or 1 inch. The calculator's trunk-and-branch method still applies to traditional tee'd systems.

Common questions

What size water line do I need for a house?
A typical 3-bedroom home with two bathrooms, a kitchen, and laundry totals roughly 30 to 40 water supply fixture units and needs a 1-inch main service line at minimum. Homes with multiple bathrooms, irrigation, or long runs from the meter often need 1-1/4 inch. The branch to individual fixtures is usually 1/2 inch; tubs and washing machines often need 3/4 inch.
What size PEX for a shower?
A single shower fixture is 2 WSFU and is served by 1/2 inch PEX in a home-run manifold system. In a trunk-and-branch layout with cumulative demand, the branch can still be 1/2 inch if static pressure at the fixture exceeds 40 psi after friction losses. Low pressure or a second-floor shower on a long run may need 3/4 inch.
What size water line for 2 bathrooms?
Two full bathrooms (toilet, lavatory, tub/shower each) plus a kitchen total roughly 18 to 22 WSFU. A 3/4 inch main trunk handles this at normal residential pressure. Each fixture branch is typically 1/2 inch; the tub or shower supply may be 3/4 inch if flow rate matters.
Is 1/2 inch PEX enough for whole house?
No. A 1/2 inch main cannot deliver adequate flow when multiple fixtures run simultaneously. Use 1/2 inch only for individual fixture branches. The main from the meter should be 3/4 inch minimum for a small home and 1 inch for most full-size houses.
What size pipe for a washing machine?
A washing machine is 2 WSFU and requires a minimum 1/2 inch supply. Many installers run 3/4 inch to the box because the fill valve can draw 4 to 6 GPM briefly, and a 1/2 inch line on a long run may not deliver that flow if someone is showering at the same time.
How many fixture units is a kitchen sink?
A kitchen sink with faucet is 1.5 WSFU under both the IPC and UPC. A dishwasher adds 1.5 WSFU. A pot filler or commercial-style faucet may be rated higher. The kitchen group typically totals 3 to 5 WSFU depending on what is connected.
What size water line for 100 feet?
At 100 feet of developed length, friction loss becomes significant. A 1/2 inch copper line loses roughly 4 to 6 psi at 5 GPM; at 1/2 inch that same flow can lose 15 psi or more. For runs over 75 feet to a fixture group, step up one pipe size — use 3/4 inch where 1/2 inch would suffice on a short run.
What is the minimum water pipe size for a toilet?
A toilet is 2.5 to 5.0 WSFU depending on the code edition and flush type. The minimum supply branch is 3/8 inch, but 1/2 inch is standard practice because 3/8 inch restricts fill rate and causes slow tank refill when pressure is marginal.
Can I use 3/4 inch PEX for main water line?
Yes, for smaller homes with moderate fixture counts. A 3/4 inch PEX main handles roughly 25 to 30 WSFU at typical municipal pressure. Larger homes, homes with irrigation, or properties with a long meter set should use 1 inch PEX or copper for the main.
What water pressure do I need for a house?
Most fixtures need 20 psi minimum at the outlet; 40 to 60 psi at the main is ideal. Below 35 psi at the street, you may need a larger pipe size or a pressure booster. Above 80 psi, a pressure-reducing valve is required to protect fixtures and prevent water hammer.

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