The water pressure has dropped.

Establish two things before anything else: is it every fixture or just one, and did it happen suddenly or gradually. Sudden and everywhere points at the supply, a valve, or a leak. Gradual and at one faucet is almost always an aerator or a cartridge, and both are a ten minute fix with no tools worth mentioning.

The four combinations

Sudden, everywhere. A valve partly closed, a supply problem on the street, a pressure regulator that has failed, or a significant leak somewhere on your side. Check the main shut-off is fully open, then check whether the neighbors have the same. If there is nothing at all, start here.

Gradual, everywhere. Usually a failing pressure reducing valve, or scale building up in old galvanized pipe. Both are real, and the second is a repiping conversation on a house of a certain age.

Sudden, one fixture. Debris in the aerator or the cartridge, often after work on the mains or after the supply has been off and back on. This is extremely common and it is why plumbers ask whether the water was recently shut off.

Gradual, one fixture. Aerator or shower head scaling up.

The five minute checks

1. The aerator. Unscrew the little screen at the end of the faucet, by hand or with a cloth for grip. Rinse the debris out, soak it in vinegar if it is scaled, and put it back. This fixes a surprising share of pressure complaints.

2. The shower head. Same idea. Unscrew, soak overnight in vinegar, refit.

3. The fixture isolation valves. Under the sink and behind the toilet. Check they are fully open. Quarter turn valves should have the handle in line with the pipe.

4. The main shut-off. Fully open. A gate valve that has been partly closed for years is a classic.

5. Hot only or cold only? If only the hot side is weak, the problem is at or after the water heater, often the shut-off valve on top of it, or sediment. The hot water sequence.

6. The pressure reducing valve, if the house has one. It is usually just after the main shut-off, a bell shaped brass fitting with an adjustment screw. They fail, and they typically fail toward low pressure. Replacing one is a plumber’s job and not an expensive one.

Measuring it, which settles the argument

A pressure gauge that screws onto a hose bib costs very little and turns this from an opinion into a number. Typical residential supply sits somewhere in the range of 40 to 80 pounds per square inch, and most fixtures are happy in the middle of that.

Two readings are worth taking:

• Static, with nothing running.
• With a faucet open, which shows whether pressure collapses under flow. Good static pressure that collapses under flow points at a restriction rather than at supply pressure.

If the reading is high rather than low, that matters too. Pressure much above 80 stresses fittings, shortens the life of appliances, and is a common cause of running toilets and dripping faucets. That is what a pressure reducing valve is for.

When low pressure is a warning rather than a nuisance

A sudden drop with a wet patch, a warm spot on the floor, or a running sound with everything off. That is a leak, and on a slab it is a specific kind. The meter test that confirms it.

A drop in freezing weather. Suspect a partly frozen line, and act before it becomes a split. What to do about a frozen pipe.

A drop with discolored water. Usually work on the mains, but also possible with a failing pipe. What to do about the color.

A drop plus a bill that jumped. Water is going somewhere it should not.

On a well

Pressure on a well system is controlled by the pressure switch and the pressure tank rather than by the street, and the symptoms are different: rapid cycling, short bursts of pressure, or a pump that runs constantly. The well sequence.

Old pipe, which is the answer nobody wants

Galvanized steel supply pipe corrodes inward over decades, and the bore narrows until flow is genuinely restricted. The tells are: an older house, gradual worsening over years, worse on the hot side, and discolored water when a faucet is first opened.

There is no cleaning that fixes it. Repiping is real work and real money, and it is worth getting a plumber to confirm the diagnosis with a look inside a cut section rather than accepting it from a sales conversation. What plumbing work costs.

The short version

Everywhere or one fixture, sudden or gradual: those two questions do most of the diagnosis. Clean the aerator and the shower head first, then check every isolation valve is fully open. Buy a hose bib gauge and take a static and a flowing reading. Sudden loss with a wet patch or a running sound is a leak, not a pressure problem.