Low water pressure is one of the most frequently misdiagnosed problems in residential plumbing. Homeowners often arrive at the conclusion that they need a new water main, when in fact a partially closed shutoff valve or a clogged aerator is the culprit. A licensed plumber will work through the causes in roughly this order of likelihood — and so should you, before paying for a service call.
Pressure versus flow
A brief distinction: pressure is the force pushing water through the system, measured in pounds per square inch (psi). Flow is the volume of water that actually arrives at the fixture, measured in gallons per minute. The two are related, but a homeowner usually notices flow first — the shower feels weak, the kitchen faucet takes too long to fill a pot. The underlying cause may be a pressure problem, a flow restriction, or both.
Normal residential pressure runs between 40 and 80 psi. A simple gauge that screws onto a hose bibb costs about fifteen dollars at a hardware store. If you have not measured your house pressure, do that before doing anything else — the reading frames every diagnosis that follows.
Cause 1: The aerator or showerhead is clogged
This is the single most common cause of "low pressure" at a specific fixture, and the easiest to fix. Sediment, mineral scale, and small particles of debris accumulate in the fine mesh of a faucet aerator or the orifices of a showerhead. Unscrew the aerator (most come off by hand or with channel-lock pliers and a rag), soak it in white vinegar for an hour, brush it with an old toothbrush, and reinstall. The improvement is often dramatic.
If only one fixture is weak and the rest of the house is normal, the cause is almost always at that fixture.
Cause 2: A partially closed shutoff valve
The main water shutoff for the house — usually located where the supply enters from the street, or near the water meter — can be partially closed without anyone noticing. This commonly happens after a plumber works on the line, after a meter replacement, or after the valve has been left unused for years and the handle no longer aligns with what the gate inside the valve is actually doing.
Find the main shutoff. Confirm that the handle (a lever) is parallel to the pipe (open), or that the wheel (a gate valve) is turned fully counterclockwise. Gate valves can also fail internally, with the gate slipping off its stem; if you suspect this, a plumber will need to replace the valve.
Cause 3: The pressure-reducing valve has failed
Most homes connected to municipal water have a pressure-reducing valve (PRV) just downstream of the main shutoff. The PRV brings street pressure — which can run 100 to 200 psi in some systems — down to a safe residential level. PRVs have a typical lifespan of ten to fifteen years and fail in two ways.
A PRV failing closed sends pressure across the entire house too low. Every fixture is weak; the toilet tank takes too long to refill; two showers cannot run together. A PRV failing open does the opposite — pressure climbs above 80 psi, faucet washers fail prematurely, and you may hear water hammer in the pipes. Replacement is a plumber job, typically $300 to $500.
Cause 4: Corroded or scaled supply pipes
In homes built before 1960, supply pipes are often galvanized steel. Over decades, the interior of these pipes scales heavily with mineral deposits and rust, narrowing the effective diameter until a half-inch pipe carries the flow of a quarter-inch one. The symptom is system-wide low flow that has worsened gradually over years. The fix is repiping — replacing the affected lines with copper or PEX — which is a significant project.
If your home has any original galvanized supply piping, a plumber can confirm the scaling by cutting a short section open. This is a useful thing to know before you spend money on other diagnoses.
Cause 5: A leak somewhere in the system
A significant leak — under a slab, in a wall, or in a buried supply line — will manifest as low pressure at fixtures because the leak is diverting flow. The diagnostic is straightforward: shut off every fixture and appliance in the house, then check the water meter. If the meter is still spinning, water is going somewhere it shouldn't.
Slab leaks and underground supply leaks are not DIY territory. A plumber with leak-detection equipment (acoustic sensors or thermal imaging) can locate the leak without tearing up the floor speculatively.
The five-minute diagnostic
Measure pressure at an outdoor hose bibb with a gauge. Check the main shutoff is fully open. Clean two aerators and a showerhead. Check the water meter with everything off. If all four pass and pressure is still low, you are almost certainly looking at a PRV or scaled-pipe issue — both of which warrant a licensed plumber.
For situations where low pressure suddenly arrives with other symptoms — discolored water, gurgling drains, or visible moisture — see What Counts as a Plumbing Emergency.
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