GPM TO PIPE SIZE
GPM TO PIPE SIZE
D ≥ √(Q ÷ 19.6) for V ≤ 8 fps
GPM
RESULT
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FILL IN ABOVE
Velocity-based sizing per industry practice: ≤8 fps cold, ≤5 fps hot. Bump up one size for hot water service. Estimate only — verify with a licensed plumber and local plumbing code/inspector before purchase or installation. Not a substitute for engineered drawings.
About this calculator
This GPM to pipe size calculator returns the smallest nominal pipe diameter that can carry a given flow rate while keeping water velocity below 8 feet per second on cold lines (5 fps on hot). Velocity above those thresholds causes pipe erosion, water hammer, and audible flow noise — issues that don't show up in friction-loss math but ruin a system. Use this for irrigation mains, hose bib supply, custom water features, mechanical room piping, and any application where you know the flow demand directly rather than summing fixture units. ESTIMATE ONLY — verify with a licensed plumber and local plumbing code before installation.
Common questions
When do I size by GPM instead of fixture units?
Fixture units (WSFU) work great for residential supply — they're probabilistic and account for fixtures rarely all running at once. GPM-based sizing is better when you know the actual continuous flow demand: irrigation zones, equipment cooling lines, hose stations, restaurant dish stations, and any system where the flow rate is dictated by the load not the fixture count.
Why is hot water sized for lower velocity than cold?
Hot water erodes copper faster — the rule of thumb is 5 fps max on hot vs 8 fps on cold. The mechanism is dezincification (in brass) and progressive copper-oxide stripping. At 140°F service temperature, sustained 8 fps flow can pinhole copper at fittings within 5–10 years. Sizing one nominal step bigger keeps velocity below the erosion threshold.
What size pipe for 10 GPM?
10 GPM cold-water service needs a minimum ¾" copper or 1" PEX line for ≤8 fps velocity. For hot at 5 fps, bump to 1" copper or 1¼" PEX. Add friction loss check separately if the run is over 100 feet — long runs may need an upsize even when velocity is fine.