Filtering at a tap has four formats and the right one is usually decided by the kitchen rather than by the water. What the spout has inside it, how much room is under the sink, and whether you own the place all narrow the choice quickly, and each format has a different running cost that is worth knowing before committing.
Tap mounted
The cheapest and fastest: the unit replaces the aerator and switches between filtered and unfiltered with a small lever. It needs a standard aerator thread, which many pull out sprays and designer taps lack, so unscrew the aerator and check before buying. Running cost is the highest per volume, because the cartridge is small and reaches its rated capacity quickly, and it will reach it much faster still if the diverter is left on filtered for washing up.
Countertop on a diverter
A larger unit on the bench connected to the tap by a diverter valve at the aerator. More media means better performance and a much longer cartridge life, so the running cost per volume drops, and it is fully removable when you move. It needs the same aerator thread as a tap mount, so it does not help where the thread is the obstacle, and it needs permanent bench space, which in a small kitchen is the real cost.
Under sink, out of the existing tap
An inline cartridge plumbed into the cold feed treats everything from the existing cold tap with nothing visible above the counter. Cartridge life is long, flow is better than a tap mount, and the kitchen looks unchanged. It needs cabinet space, a tee into the cold supply and clearance to change the cartridge. The compromise is that all the cold water is filtered, including what fills a washing up bowl, which consumes the cartridge faster than a dedicated tap would.
A dedicated filtered tap
The neatest and the most work: a separate small tap on the sink deck fed from an under sink system, so only water you actually drink is filtered. Cartridge life is the longest of the four for that reason. It is required for reverse osmosis, since RO water should not share a path with unfiltered hot water. It needs a spare deck knockout or a hole drilled through the worktop, easy in stainless and a specialist job in stone. Our faucet filter guide compares certified units across these formats.
Questions people ask about filter for faucet
Which format is cheapest to run?
A dedicated filtered tap, because only drinking water passes the cartridge. Tap mounted units cost the most per volume treated.
What if my tap has no aerator thread?
Tap mounted and countertop diverter formats are both ruled out. An under sink cartridge or a dedicated tap are the alternatives.
Which suits a rental?
Tap mounted or countertop, since both attach at the aerator thread and leave no trace when removed.
Do I need a separate tap for reverse osmosis?
Yes, so treated water never shares a path with unfiltered hot water through the mixer.