What size water supply line does a house need?

Total your water-supply fixture units, convert to peak GPM on the Hunter curve, then read the pipe band. A common result is a 3/4-inch trunk with 1/2-inch branches to fixtures. Around 20 fixture units (~14 GPM) fits 3/4-inch. Run your numbers in the supply pipe size calculator.

Why are pipes sized by fixture units instead of fixture count?

Because fixtures rarely all run at once. Fixture units weight each fixture by flow and how often it is used, and the Hunter curve converts the total to a realistic probable peak — demand does not double when fixture count does.

How many GPM is 20 fixture units?

About 14 GPM of probable peak demand; 30 fixture units is roughly 18 GPM. The conversion flattens as the count grows. See the peak-demand GPM calculator.

Should I size up for a long run?

Yes. Long runs lose pressure to friction and height costs about 0.43 psi per foot, so a pipe that is adequate at the meter can starve a distant or upstairs fixture. Check with the friction loss tool and size up a step if needed.

How to Size Water Supply Pipes

Supply pipes are not sized by how many fixtures they feed but by how much water those fixtures are likely to demand at the same time. That is what fixture units capture — and why a house with ten fixtures does not need ten fixtures’ worth of pipe.

By Francesco Zinghinì

Why not just count fixtures?

Fixtures almost never all run at once. A toilet fills for a minute, a shower runs for ten, a hose bibb is used rarely. Plumbing codes handle this with water-supply fixture units (WSFU) — a weighting that reflects each fixture’s flow and how often it runs. Summing WSFU and converting to a probable peak flow gives a realistic design number instead of the impossible worst case of everything open together.

Step 1: total the fixture units

Assign each fixture its WSFU value and add them up. Typical values: a tank toilet 2.2, a bathroom sink 0.7, a shower 1.4, a kitchen sink 1.4, a clothes washer 1.4, a hose bibb 2.5. The water-supply fixture units calculator totals them from your fixture counts, and the full list lives on the fixture units table.

Worked example. A toilet (2.2), a sink (0.7), a shower (1.4), a kitchen sink (1.4) and a laundry hookup (1.4) total 7.1 WSFU for a small bathroom-plus-utility group.

Step 2: convert WSFU to peak GPM

Fixture units are not gallons per minute — they convert to a probable peak flow through the Hunter curve, which flattens as the count grows (the more fixtures, the lower the chance they all run together). The peak-demand GPM calculator does this conversion.

Worked example. About 20 WSFU maps to roughly 14 GPM of probable peak demand; 30 WSFU to about 18 GPM. Notice the curve: demand does not double when fixture units do, because simultaneity falls.

Step 3: read the pipe size for that flow

Now pick the smallest pipe that carries that GPM at a sane velocity (roughly 5–8 ft/s — go faster and you invite noise, erosion and water hammer). The supply pipe size calculator reads it off a capacity band, and the supply pipe capacity table lists the bands: about 9 GPM for 1/2-inch, 18 GPM for 3/4-inch, 33 GPM for 1-inch.

Worked example. The 20-WSFU group at ~14 GPM fits within the 3/4-inch band (up to ~18 GPM), so 3/4-inch copper is the typical trunk size — while individual fixture branches drop to 1/2-inch. That is the usual pattern: a 3/4-inch main artery with 1/2-inch branches to fixtures.

Length and pressure matter too

Diameter is only half the story. A long run loses pressure to friction, and every foot of height above the meter costs about 0.43 psi. If the supply is already weak or the run is long, size up a step, because a pipe that is technically adequate at the meter can starve a top-floor shower. Estimate the losses with the friction loss and pressure and head calculators before you finalize.

Velocity: the ceiling you must not exceed

You could push more flow through a small pipe by accepting higher velocity — but excessive velocity is what erodes copper, hammers on shutoff, and makes pipes sing in the wall. The capacity bands already build in a reasonable velocity ceiling, which is why you size up to meet flow rather than squeezing flow through a smaller pipe.

Material changes the capacity

The pipe bands assume a reasonable velocity, but the material still matters. Copper and CPVC are sized on their nominal diameter; PEX has a smaller inside diameter than copper of the same nominal size, and its insert fittings restrict flow further, so many plumbers size PEX up a step for the same demand — a 3/4-inch copper trunk may become a 1-inch PEX trunk to carry the same GPM comfortably. Smooth plastic also has lower friction loss than rough old metal, which is one reason a fresh PEX or CPVC repipe can restore pressure that a corroded galvanized system had lost. When you read a capacity band, treat it as the starting diameter for copper and check whether your material calls for a size up. Home-run (manifold) PEX layouts change the picture again by giving each fixture its own line, trading more tubing for steadier pressure when several fixtures run at once.

Common sizing mistakes

Sizing by fixture count. Ten fixtures do not need ten fixtures’ worth of pipe — use fixture units and the Hunter conversion, or you will grossly oversize.
Ignoring the run length. A diameter that is fine at the meter starves a far or upstairs fixture once friction and height are counted; check with the friction loss and head tools.
Forgetting the PEX downsize. Matching PEX to copper nominal size without accounting for its smaller bore can leave a branch short on flow.
Undersizing the trunk to save money. The main artery carries the whole peak; skimping there causes pressure to sag every time a second fixture opens — the low-flow complaint from the pressure guide.

Labeled bands, not code

The WSFU values, the Hunter conversion and the capacity bands here are widely used planning conventions, not a code citation. Sizing methods, allowed materials and velocity limits vary by jurisdiction, and your local plumbing code and inspector govern. Use these tools to plan and sanity-check, then confirm the design with a licensed plumber and pull the required permit.