Why is my water pressure suddenly low?

Before anything else, walk the house and run taps. Where the pressure drop shows up tells you what it is, and the two answers lead to completely different places.

If it is one fixture

Good news — this is usually a fifteen-minute fix with no specialist tools. The most common cause is a clogged aerator: the little screen that threads onto the end of a faucet spout. Unscrew it, and you will typically find it packed with mineral scale and grit. Soak it in vinegar, scrub it, reinstall. On a showerhead, the same logic applies to the spray face. If the aerator is clean and pressure is still low, the fixture’s cartridge or supply valve is next, and both are common replacement parts.

If it is hot water only

That points at the water heater rather than the plumbing — often sediment restricting the outlet, or a partially closed valve on the hot side. Worth a flush.

If it is the whole house

• Check the main shutoff valve is fully open. If anyone has worked on the plumbing recently, a partially reopened valve is a very common and very anticlimactic cause.
• Check the pressure-reducing valve, usually located where the main line enters the house. These fail over roughly 10–15 years and failing low is a normal way for them to go.
• Ask a neighbour. If their pressure dropped too, it is the municipal supply and there is nothing to fix on your end.
• On a well system, suspect the pressure tank or its switch.

The one that matters

A sudden whole-house pressure drop with no obvious cause can indicate a leak in the supply line — possibly underground and possibly running continuously. Two things to check: the water meter with every fixture in the house off (any movement means water is going somewhere), and unusually green or soggy patches in the yard along the supply line. If either shows up, that is a call-today situation, not a weekend project.

Barring those signs, low pressure is more often annoying than urgent — but it is worth knowing which of the two you have before deciding to live with it.