TradesHow this works
Sizing a water line is a balance between two things. Make the pipe too small and the water moves too fast and loses too much pressure on the way, so the fixture at the end dribbles. Make it too big and you have paid for copper or PEX you did not need. This tool finds the smallest pipe that still delivers the pressure you need at the far fixture while keeping the water speed in a sensible range.
It works out the friction loss with the Hazen-Williams method, adds any pressure lost climbing to an upper floor, and checks what is left at the outlet. It also checks velocity, because a line can pass on pressure and still be too loud and too hard on the pipe. When both checks pass, that is your size.
A worked example
Say you need 8 gallons per minute at a bathroom group, you have 40 psi at the meter, and the fixtures want about 30 psi to work well. On a 40 foot run, three quarter inch copper carries that fine. Stretch the same run to 300 feet and the friction eats past your 10 psi of headroom, so the tool steps you up to one inch. Same flow, same pressure at the street, but the distance is what forced the bigger pipe.
What it does not do
It does not size drains or vents, it does not apply a specific jurisdiction's fixture-unit tables, and it does not certify a design. Use it to reason about a supply line and confirm against the code where you work.
Common questions
Does this replace a plumber or the plumbing code?
No. It is a planning tool. It helps you reason about a pipe size and see the pressure and velocity, but it does not certify anything and it does not replace a licensed plumber, a permit, or the code that applies where you work. Codes differ, and some follow the IPC while others follow the UPC.
GPM or fixture units?
If you know the peak flow, enter it in GPM. If you only have a fixture count, leave GPM at zero and enter the total water supply fixture units. The tool estimates the likely demand from that using an approximation of the standard Hunter curve. A measured GPM is always more accurate.
Why does velocity matter, not just pressure?
Water moving too fast is loud, it can erode copper over time, and it causes water hammer. A common rule is to keep cold water under about 8 feet per second and hot water under about 5. The tool flags a size that pushes past your velocity target even if the pressure still works.
Copper or PEX?
PEX is cheaper and installs fast with no soldering, but its inside diameter is smaller than copper for the same nominal size, so it runs a little faster and loses a bit more pressure. Copper has a larger bore and handles heat well. Pick "Compare both" to see them side by side for your run.
Sources: Hazen-Williams friction method (US customary) · Copper Type L dimensions (ASTM B88), PEX (ASTM F877) · Hunter fixture-unit demand curve (approximation). Engine version 1.0.0. Risk tier 2.