The right way to filter water is to find out what is in it first, then match the media to the problem. On city water that means reading your utility's annual water quality report; on a well it means a lab test. Once you know the contaminants, the choice between a pitcher, an under-sink unit and a whole-house system becomes a straightforward decision instead of a guess.
Start with what is actually in your water
Every community water system in the US publishes an annual Consumer Confidence Report describing what was detected in the supply, and the EPA maintains a finder to locate yours. That report tells you whether you are dealing with chlorine taste, disinfection by-products, or something that needs more serious treatment. Well owners have no such report and should start with a certified lab test instead. Buying filtration before reading the evidence is how people end up treating problems they do not have while missing the one they do.
Point of use or point of entry
Point-of-use filtration treats one outlet: a pitcher, a faucet unit, an under-sink system or a countertop machine serving drinking and cooking water. Point-of-entry treats the whole house, so showers, laundry and the water heater all benefit. Taste and drinking-water contaminants are usually handled cheapest at the tap you drink from, while chlorine you can smell in the shower, sediment wearing on appliances, or staining minerals justify a whole-house approach. Our whole house water filter guide walks through the point-of-entry side in detail.
Match the media to the problem
Each technology has a lane. Sediment cartridges strip sand, rust and grit. Activated carbon adsorbs chlorine, many taste and odor compounds and a range of organic chemicals. Reverse osmosis pushes water through a membrane and rejects most dissolved solids, which is why it is the usual answer for things carbon cannot touch. UV disinfection neutralizes bacteria and other microbes but removes nothing physical. Serious well problems are usually solved with a train of several of these in the right order rather than any single box.
Questions people ask about how to filter water
Is boiling water the same as filtering it?
No. Boiling kills microbes but removes nothing dissolved and can slightly concentrate minerals as water evaporates. Filtration and boiling solve different problems.
Is a filter pitcher enough for most homes?
For chlorine taste and odor on treated city water, often yes. It is not the tool for dissolved solids, bacteria or well water problems, which need RO, UV or a whole-house train.
Do I need a whole house filter or just one at the sink?
Treat drinking-water issues at the sink and whole-body or appliance issues at the point of entry. If chlorine bothers you in the shower or sediment is wearing fixtures, go whole house.