Pipe Sizes and Why They Affect Your Water Pressure

Pressure and flow are not the same thing

People describe both as water pressure. A gauge on an outside tap measures static pressure with nothing running. What you actually notice at a shower is flow, which is how much water arrives per second while it is running.

A house can have perfectly adequate static pressure and still deliver a disappointing shower, because pressure is lost to friction as water travels through pipe. The narrower and longer the run, and the more fittings and bends it passes through, the more is lost.

Why diameter matters more than it looks

The relationship is not proportional. Reducing pipe diameter reduces carrying capacity far more sharply than the change in size suggests, because capacity relates to cross-sectional area rather than width. A modest reduction in internal diameter produces a large reduction in flow.

That is why a service line one size too small is felt immediately when two fixtures run at once, and why the same house is fine when only one is open.

Corrosion does the same thing as undersizing

This is the part that catches people out. Galvanized pipe narrows internally as it corrodes and scales. The pipe is nominally the right size and behaves like a smaller one. From the tap, an undersized line and a corroded line feel identical, and the two are regularly confused.

Telling them apart matters, because one is a design problem and the other is a deteriorating pipe that will keep getting worse.

What we look at

Where pressure drops, and whether it drops everywhere or at one fixture. Whether it worsens when a second outlet opens. What the supply material is and how old it is. Static pressure at the hose bib as a baseline.

One fixture affected usually means that fixture. The whole house affected, gradually worsening over years, usually means the supply lines themselves.