The question that solves half of it
Where is the pressure low? One fixture means that fixture: a clogged aerator or a failing cartridge, both cheap fixes. Hot side only points at the water heater or its lines, often sediment. Everywhere in the house, hot and cold alike, means the problem is upstream of everything: the main valve, the pressure regulator, the service line, or the city supply.
This one distinction saves more wasted effort than any tool. Don’t touch the regulator until you’ve confirmed it’s a whole-house problem.
The 15-minute diagnosis
In order, each step rules out the cheapest remaining explanation.
Unscrew and clean one aerator
The screen at the faucet tip catches grit and scale. Soak it in vinegar, rinse, re-test. If flow is restored, do the rest of the house and you’re done.
Verify the main valve is fully open
Both the house-side shutoff and the meter valve. A valve someone half-closed during a past repair is a classic hidden cause.
Gauge a hose bib
A $12 pressure gauge threads onto any outdoor faucet. Healthy homes read roughly 45–80 psi. Below about 40, the regulator or supply is the story.
Ask a neighbor
If their pressure is low too, it’s the utility, call them, not a plumber. If they’re fine, the problem is on your side of the meter.
Suspect the regulator, or the pipes
A failing pressure-reducing valve is a straightforward replacement. But if your home has old galvanized pipe, decades of internal rust may be the real bottleneck, and that’s a repipe conversation, not a valve swap.
“Low pressure at one faucet is a five-dollar fix. Low pressure everywhere is a system telling you where to look.”
The bottom line
Run the checklist before anyone sells you anything. If it ends at the regulator, that’s a routine replacement; if it ends at galvanized pipe, get the assessment, pressure that’s been fading for years usually isn’t coming back any other way.