Water filters for kitchen sinks come in four formats: under-sink cartridges plumbed into the cold line, faucet-mounted units on the spout, countertop filters fed by a diverter hose, and under-sink reverse osmosis. They differ far more in what they remove and how much cabinet work they need than in how they look.
Under-sink cartridge filters
These tee into the cold supply inside the cabinet and feed either a separate small faucet or, in some designs, the main cold tap. The cartridge is large, so contact time with the carbon is good and the certified claims are broader than a faucet unit's. Installation means shutting off the stop valve, fitting a tee and drilling the countertop or reusing a spare sprayer hole. Changing the cartridge is a job you do kneeling in the cabinet, so leave clearance below the housing when you mount it.
Faucet-mounted and countertop filters
Both borrow the aerator thread, so both are renter-friendly and need no plumbing. A faucet unit hangs on the spout with a bypass lever; a countertop unit sits beside the sink with a hose. Neither can be fitted to a pull-down sprayer, and both slow the flow while filtering. Their cartridges are smaller, so they suit chlorine taste, odor and particulates well and specific dissolved contaminants less well. Check the certification for the exact claim you care about before buying either.
Reverse osmosis under the sink
Reverse osmosis is the format to choose when your problem is dissolved: nitrate, arsenic, sodium, fluoride or a high total dissolved solids figure that carbon cannot touch. It needs cabinet room for a multi-stage manifold and a small storage tank, a drain saddle on the waste pipe and a dedicated faucet. It also sends a continuous drain stream while it produces. If your complaint is only that the water tastes of chlorine, a carbon cartridge does that job with far less hardware.
Choose from a report, not from a taste
The EPA requires every community water system to publish an annual Consumer Confidence Report and maintains a finder for them, so if you are on city water you can read what is actually in your supply before spending anything. Match the format to what the report shows: carbon for disinfection byproducts and taste, a certified lead claim for lead, reverse osmosis for dissolved solids. Our replacement water filters guide covers keeping whichever you choose in service afterwards.
Questions people ask about water filters for kitchen sinks
Which kitchen sink filter format removes the most?
Under-sink reverse osmosis, because it handles dissolved contaminants that carbon cartridges leave in place. It also needs the most cabinet space and a drain connection.
Can I fit a kitchen sink filter as a renter?
Yes, with a faucet-mounted or countertop unit. Both use the aerator thread and come off without a trace, provided your faucet has a usable thread.
Do kitchen sink filters need a second faucet hole?
Under-sink and reverse osmosis units normally do, though a blanked sprayer or soap dispenser hole often serves. Faucet and countertop filters need no hole at all.