How to size a water line: pipe sizing basics
Get the pipe diameter right and every fixture gets steady pressure; get it wrong and the shower goes cold every time someone flushes. Here is how to size a water line, step by step.
Learning how to size a water line comes down to matching the pipe to the water it has to move. Too small and you starve the fixtures; too big and you waste money and let water sit. The size depends on four things: how much water the line serves (demand), how far it travels (length), how hard the water is pushing (pressure), and how fast you let it flow (velocity). Get those four right and the pipe size follows. Results here are estimates for guidance, not a substitute for local plumbing code.
Step 1 — Total the fixture units
Every fixture is rated in water supply fixture units (WSFU) — a lavatory is small, a tub or washing machine is larger. Add up the WSFU for everything the line feeds. Fixture units are the standard way plumbing code expresses demand, because plumbing fixtures rarely all run at once.
Step 2 — Convert fixture units to demand (GPM)
Convert the total WSFU into peak demand in gallons per minute (GPM) using a demand (Hunter's) curve. The curve accounts for the fact that as you add fixtures, the odds of all of them running together drop — so demand rises fast at first, then flattens. This GPM figure is what your pipe actually has to carry.
Step 3 — Note length and available pressure
Measure the developed length (the real run, including fittings) and your available pressure at the source. Longer runs and more fittings lose more pressure, so a line that is fine at 20 feet may need to step up a size at 80 feet. Aim to keep flow velocity around 5 to 8 feet per second — faster than that gets noisy and erodes pipe over time. For the underlying flow relationships, references like the Engineering ToolBox tabulate the same values.
Step 4 — Pick the diameter
Choose the smallest diameter that carries your GPM within the velocity and pressure-loss limits. That is exactly what the pipe diameter calculator and pipe sizing calculator do for you — enter demand and length, get the size. Cross-check flow with the water flow rate calculator.
Typical residential water line sizes
| Line | Common size |
|---|---|
| Main service (street to house) | 3/4″ – 1″ |
| Main trunk / risers | 3/4″ |
| Branches to fixtures | 1/2″ |
| High-demand runs (multiple baths) | 1″ + |
These are starting points, not a substitute for sizing to your actual load. A big house with several bathrooms on one trunk will need to step up.
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PlumbCalc Pro turns demand and length into the right diameter — free, in your browser.
Open PlumbCalc Pro ▶ Subscribe on YouTubeHow to size a water line — frequently asked questions
How do you size a water supply line?
Total the fixture units, convert to GPM demand, then pick the smallest diameter that carries it within safe velocity and pressure loss over the run.
What size water line for a house?
Typically a 3/4″ or 1″ main, 3/4″ trunk, and 1/2″ branches — but size to your real fixture load and local code.
What if a water line is undersized?
Pressure drops when several fixtures run at once. Sizing to real demand avoids both weak pressure and the waste of oversizing.
Is PlumbCalc Pro free?
Yes — the core calculators are free with no signup; an optional one-time PRO purchase unlocks advanced tools.