Water filters under the sink come in three basic designs: a single carbon cartridge plumbed into the cold line, a multi-stage cartridge system with its own faucet, and a reverse osmosis unit with a membrane and drain line. They differ sharply in what they remove, so the right pick starts with what is actually in your water, not with cabinet dimensions.
Single-stage carbon: the taste fixer
One carbon block cartridge inline on the cold supply feeds your existing faucet. It improves taste and odor, cuts chlorine, and depending on the cartridge's certification can reduce lead and some other contaminants at the tap. It is the cheapest and simplest under-sink option, changes in minutes, and needs no extra faucet hole. Its limit is dissolved solids: carbon does not touch hardness, nitrates or total dissolved solids, so it is a taste upgrade rather than a full treatment.
Multi-stage systems: targeted stacks
Multi-stage units chain two or three cartridges, typically sediment, carbon and a specialty stage, and usually feed a dedicated drinking faucet mounted at the sink. The advantage is targeting: you can slot in a lead-specific, fluoride-specific or chloramine-specific cartridge where a single stage cannot. The cost is more cartridges on more schedules and a hole drilled for the faucet. Check your annual Consumer Confidence Report before choosing the specialty stage; the EPA maintains a finder for every community system's report, and the report tells you what is worth targeting.
Reverse osmosis: the deep clean
An RO unit adds a membrane that rejects dissolved solids wholesale, which is territory no carbon cartridge reaches. It needs a drain connection for the reject stream and either a storage tank or a pump, so it is the most involved install of the three. Choose it when your report or lab test shows dissolved contaminants, or when you want the most thorough drinking water available at one tap. Whichever design you land on, our under-sink water filter installation guide shows what the plumbing actually involves.
Questions people ask about water filters under the sink
Do under-sink filters treat the whole kitchen tap flow?
Inline carbon units do, at the cost of faster cartridge wear. Multi-stage and RO systems feed a separate low-flow drinking faucet and leave the main tap untreated.
How often do the cartridges need changing?
Follow each cartridge's rated capacity and the maker's schedule. Taste returning, flow dropping or a due-date passing all mean change it now; an expired cartridge can be worse than none.
Which design handles lead best?
Both a lead-certified carbon cartridge and a reverse osmosis system address lead at the tap. If lead is your driver, verify the specific model's certification rather than the category.