Pressure that drops a little at a time is easy to ignore. The shower takes longer to fill, the kitchen faucet sprays less, and after a while everyone in the house has adjusted without noticing. A gradual loss usually points to something specific, though, and the cheapest fix often depends on finding which fixture or which part of the system is restricting flow. Sort the problem before you buy parts or call anyone.
Check whether the loss is everywhere or only in one place
Run cold water at an outdoor spigot or a laundry tap, then at a kitchen faucet, then at a bathtub. If every outlet is weak, the restriction is probably upstream of the branches, which points toward the main supply, the meter area, or a treatment unit on the main line. If only one fixture is weak, start there. A clogged aerator or showerhead screen causes this more often than people expect, and it is quick to test: unscrew the outlet, look at the screen, and run the water with it removed. If the flow comes back, the problem was local.
Compare the readings on each side of the filter
If the house has a whole-house filter or softener on the main line, a pressure gauge on the inlet and another on the outlet show what the unit is doing. Take both readings while water runs at a steady draw, such as a hose or a tub filling, and write down the day you took them. A growing gap between the two numbers means the media or cartridge is catching material and resisting flow. A small gap with low pressure on both sides means the problem sits upstream of the unit, and the filter is probably not the cause. Repeat the readings every few weeks so you can see a trend instead of a single moment.
Look at the pattern before you touch anything
Ask a few questions first. Did the drop start after a utility shutdown, a street repair, or a plumbing job at the house? Did it follow any work on the treatment equipment, such as a cartridge change or a bypass valve that may have been left partly open? Is the weakest flow at the start of a draw and better after a minute? A restriction that builds in a cartridge usually shows as a steady, slow loss. Sediment that breaks loose may cause a short burst of cloudiness followed by recovery, which is a different pattern and worth noting.
Ask the installer or service company what they measure
If a company set up the equipment, ask what the normal pressure drop across the unit is at a stated flow rate, and what reading means it is time to service it. Ask which parts they expect to wear, how the cartridge or media is accessed, and whether the unit was sized for the household's busiest hours. A clear answer names numbers and a service interval. A vague answer tells you something too, and it is useful when you compare quotes. For a sense of how the companies on this site are evaluated, see our ranking methodology.
What to do before you call anyone
Write down which outlets are weak, what the unit readings were, and what changed before the drop began. Clean the fixture screens, confirm the bypass valve is fully in service position, and check that the filter housing is seated and not leaking. If the readings show a growing gap across the unit, plan a cartridge or media inspection, and ask the technician to show you what they found. If the whole supply is weak and the filter readings are normal, the cause is outside the treatment equipment, and the next call is to a plumber or the water utility rather than a filter service.