US water filters fall into two families: point of use units that treat one tap and point of entry units that treat the whole building. Which family you need depends on whether your problem is the water you drink or the water that runs through your plumbing, and certification is how you check the claim on the box.
Know your supply before you shop
Roughly one household in seven in this country draws from a private well, and those wells are not regulated under the federal Safe Drinking Water Act, so the owner is responsible for testing and treating. If you are on a community water system, the utility publishes an annual Consumer Confidence Report describing what it delivers, and the EPA maintains a finder to locate yours. Those two starting points lead to different shopping lists: city households usually target chlorine, taste, lead and specific contaminants, while well households more often face iron, hardness, sediment, sulfide and bacteria.
Point of use and point of entry
A point of use filter sits at a single fixture: a pitcher, a faucet mount, a countertop unit, an under sink cartridge system or a reverse osmosis unit. Because it treats a small volume slowly, it can use fine media and membranes that would strangle a whole house. A point of entry system sits on the main line and treats everything, which is what protects the water heater from scale and sediment and stops staining in the tub. Households with both a drinking water concern and a plumbing concern normally end up running one of each rather than choosing.
Certification is how a claim gets checked
Independent certification to the recognized drinking water treatment standards is what separates a tested performance claim from marketing copy. Different standards cover aesthetic effects like chlorine, taste and particulates, health related reduction claims such as lead and certain organics, reverse osmosis systems as a category, and emerging contaminant claims. Look for the specific contaminant listed on the certification, not just a certification mark on the box, because a unit can be certified for chlorine reduction and have no tested claim at all for the contaminant you actually care about.
Matching media to the contaminant
Sediment filters catch grit, rust flakes and turbidity and protect everything downstream. Activated carbon covers chlorine, taste, odor and many organics, and catalytic grades handle chloramine. Ion exchange removes hardness and, with the right resin, specific ions such as nitrate. Reverse osmosis is the broadest single answer for dissolved contaminants at a tap. Oxidizing filters handle iron and sulfide on wells. Ultraviolet lamps disinfect clear water. No single technology covers everything, which is why treatment is built as a train in sequence rather than a single box.
Buying with replacement filters in mind
The purchase decision that costs the most over time is not the housing, it is the cartridge supply behind it. Systems using standard sized cartridges can be resupplied from many makers, while proprietary shapes lock you to one seller for the life of the unit. Check availability and rated capacity before you commit, and keep the model numbers somewhere you will find them. Our replacement water filters page covers matching cartridges to housings, which is how most households avoid paying a premium every time a filter is due.
Questions people ask about us water filters
Are water filters regulated in the United States?
Filters themselves are not federally approved; performance claims are verified through independent certification to recognized drinking water treatment standards, and buyers check the certified contaminant list.
Does city water need filtering at all?
It is treated and monitored before delivery, so filtering is usually about taste, chlorine, hardness or a specific concern such as lead from household plumbing, rather than about basic safety.
Which filter is best for a private well?
There is no single answer, because well problems vary by geology. Test first, then build a train for what the test finds, typically sediment, iron treatment, softening and disinfection in that order.
Do I need both a whole house filter and an under sink filter?
Many households do. The whole house unit protects plumbing and appliances, while the under sink unit uses finer media to polish the small volume actually used for drinking and cooking.