The International Plumbing Code (IPC) requires a minimum of 20 PSI at any fixture. Practically, most fixtures perform best at 30–60 PSI. Tankless water heaters often need at least 30–40 PSI to activate.

The C value (roughness coefficient) represents how smoothly water flows through a pipe. Higher C = smoother pipe = less friction loss. PEX (C=150) and copper (C=140) are very smooth, while old galvanized steel (C=100) is much rougher and causes more pressure drop.

Each additional fixture increases the total flow rate (GPM) through your supply pipes. As flow rate rises, friction loss increases exponentially (to the power of 1.85), causing noticeable pressure drops at all open fixtures simultaneously.

If you have a pressure-reducing valve (PRV), you may be able to adjust its set screw to increase pressure (up to the supply main pressure). If you have no PRV and low city pressure, a booster pump is the solution. Always stay below 80 PSI to protect fixtures and water heaters.