Pipe size calculator

Because it has a much smaller hole for the same name. PEX is made to SDR 9 on a copper tube size outside diameter, so 1 in PEX has a 0.875 in bore against 1 in Type L copper's 1.025 in — 27% less flow area. Run 8 gpm through both at 8 ft/s and 4 psi per 100 ft and copper passes at 1 in while PEX has to go to 1-1/4 in. Insert fittings make it worse still, because an F1807 insert reduces the bore again at every joint, which is why expansion and press systems flow better at the same nominal size.

No, and that is deliberate rather than an omission. Converting fixture units to a design flow is Hunter's curve, published as IPC Appendix E, and this site does not reproduce that table — the figures could not be established with enough confidence, and a wrong design flow sizes the whole system rather than one number. Get the gpm from the code table your jurisdiction adopted, or from a demand study, and bring it here.

Two to three feet per second, and it is the strictest number on the list for a physical reason rather than a comfort one. Fast hot water strips the protective cuprous-oxide film off the bore of a copper tube, the exposed metal re-oxidizes, and the cycle removes wall — erosion-corrosion. A return line runs hot every hour of every day, so it accumulates that damage at a rate a branch never does. Pinholes on the outside of elbows in a recirculating hot line are this failure and essentially nothing else.

Because it changes the viscosity, and the viscosity sets the friction factor through the Reynolds number. Hot water is thinner, so it loses less head — around 14% less at 180 °F than at 55 °F on the same pipe and flow — which occasionally lets a run stay one size down when the friction budget is the binding limit. It works the other way on hot copper, where the velocity ceiling drops from 8 ft/s to 3 and pushes the size back up. Both effects are live in the answer at the same time.

It means you have run off the end of the table rather than that the answer is the biggest size shown. PEX above 2 in leaves copper tube dimensions and moves to iron pipe size, and CTS CPVC stops at 2 in altogether, so both families genuinely end there — the honest move is to change material, split the duty across parallel runs, or revisit the limits. The comparison table shows which of the five gets there, which is usually steel or PVC schedule pipe.

No — oversizing has its own failure modes and they are not financial. Water sitting in an oversized run is water with a long residence time, which is a legionella and a disinfectant-decay problem in a building; an oversized hot branch also means a longer wait at the tap and more heat dumped into the wall on every draw, because the slug of cooled water ahead of the hot has to be pushed out first. Below about 5 ft/s in a distribution main you are usually paying for pipe that is buying you nothing.

No, and no calculator that tells you otherwise has read the code. Neither the IPC nor the UPC states a numeric maximum velocity for water distribution in the body of the code — both require sizing by an approved method, and the approved methods carry these figures. The copper values come from the Copper Development Association's handbook, the suction and discharge figures from Hydraulic Institute practice, the 5 ft/s thermoplastic figure from plastic pipe industry guidance, and the customary 5 to 8 ft/s band from ASPE design practice.