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Water Pipe & PEX Sizing Calculator

Enter the flow rate and whether it's a hot or cold line — get the minimum recommended pipe diameter, rounded to a standard PEX size.

The professional method this calculator simplifies

Plumbing codes (UPC/IPC) actually size pipe using Water Supply Fixture Units (WSFU), not raw GPM directly — each fixture is assigned a fixture-unit value (a toilet is roughly 3 WSFU, a shower around 2, a sink around 1), the units for all fixtures on a line are summed, and that total is converted to a design GPM through Hunter's Curve, a statistical table that accounts for the fact that not every fixture on a line runs at full flow simultaneously — the same "not everything runs at once" logic behind the electrical service calculator's demand factor. This calculator skips straight to a GPM estimate for simplicity; a full WSFU calculation is more accurate for a line serving many fixtures, since simple GPM addition increasingly overstates real demand as fixture count grows.

How to use this calculator

Add up the flow rate (GPM) of fixtures expected to run simultaneously on this line, and pick hot or cold — each has a different maximum design velocity in most plumbing codes. The calculator returns the minimum diameter that keeps flow at or under that velocity, rounded up to a standard PEX size.

Why hot and cold lines size differently at the same flow rate

Cold water lines are conventionally designed for up to 8 feet per second; hot lines are held to a lower limit, since higher velocity in hot water accelerates pipe erosion and noise (water hammer) more than it does in cold lines. The same GPM demand on a hot line needs a larger pipe than on a cold line to stay under its lower velocity limit.

The two major US model codes don't fully agree on that hot-water limit: the Uniform Plumbing Code (UPC) allows up to 5 fps, while the International Plumbing Code (IPC) caps it at 4 fps. This calculator defaults to the stricter 4 fps figure so the result is safely sized under either code — if you know you're working under UPC specifically and want the less conservative number, you can size slightly smaller than what's shown here, but check with your local jurisdiction first since adopted codes (and local amendments) vary by state and even by city.

Worked example

8 GPM on a cold line (8 fps max): the calculated minimum diameter is about 0.78 inches, which rounds up to a standard 1-inch PEX line.

Frequently asked questions

How do I estimate GPM if I don't know fixture flow rates?

A rough reference: shower ~2.5 GPM, bathroom sink ~1.5 GPM, dishwasher ~1.5 GPM, washing machine ~2 GPM, toilet fill ~3 GPM (brief). Add up whichever fixtures could realistically run at the same time on this specific line.

Does pipe material (PEX vs copper vs CPVC) change the sizing math?

The core velocity-based sizing approach is the same across materials — what differs slightly is each material's actual internal diameter at a given nominal size, and PEX in particular tends to have a somewhat smaller internal diameter than copper at the same nominal size due to wall thickness.

Should the whole house main be sized the same way as a branch line?

Yes, same method — just with a higher total GPM, since the main has to carry the combined demand of every branch it feeds. Main lines are typically 3/4" to 1" in a standard single-family home.

Reference only — verify against your local plumbing code (UPC/IPC) and consult a licensed plumber before installing or replacing water piping.