Plumbing Physics: Pipe Sizing, Velocity, and Pressure Drop
Why does your shower pressure drop when the toilet flushes? It is all about Flow Rate vs. Velocity. We explain pipe sizing physics, the difference between PEX and Copper, and how to maintain pressure.
Plumbing is fluid dynamics. The goal is to deliver water quietly, at the right pressure, without wearing out the pipes.
Flow Rate (GPM) vs. Velocity
Water velocity should generally be kept below 5-8 feet per second. Too fast, and you get 'water hammer' (banging pipes) and erosion corrosion (water wearing away the copper). Too slow, and you get sediment buildup.
Pipe Material: Copper vs. PEX
Copper (Type M, L, K): Rigid, durable, naturally antimicrobial. Expensive. Subject to freezing bursts.
PEX (Cross-linked Polyethylene): Flexible, freezes without bursting, cheap. Connections (crimp vs expansion) restrict flow slightly more than copper.
Pressure Drop
Every foot of pipe and every elbow adds friction, which lowers pressure. This is Friction Head Loss. If you run 1/2 inch pipe for 50 feet to a second-story shower, you might lose too much pressure. Upgrading to 3/4 inch pipe reduces friction by 75%.
Try it yourself
Calculate flow rate and pipe volume:
Proper sizing prevents noise and ensures a strong shower spray.