Tap mounted filters look alike and are sold on much the same language, so it helps to have a short list of things that actually differ between them. Four questions cover it, and answering them in order will usually leave one or two candidates rather than a shelf of options that all claim to remove everything.
One: what is it certified to reduce
Independent certification is the only claim that can be verified, and certifiers publish listings naming the model and the exact contaminant reduction claims tested. Most of these units are certified for chlorine taste and odor. Fewer are certified for lead, cysts or specific chemicals, and those that are will say so in the listing rather than only on the box. Decide what you need reduced first, then filter the options by what has actually been tested, which usually shortens the list considerably.
Two: how much water will it treat
Capacity is stated as a rated volume, and small cartridges reach it faster than people expect, particularly if the diverter is left on filtered for washing up. Divide the rated volume by your household's realistic drinking and cooking use to get a genuine interval, then compare cartridge prices on that basis rather than on the price of the unit. A cheap filter with an expensive short lived cartridge is the more costly purchase over a year, and cartridge availability in a few years matters too.
Three: flow, and what it does to the kitchen
All of these units restrict flow, and the restriction grows as the media loads. On a supply with modest pressure the filtered setting can feel slow enough to be irritating for filling a pan, which is the most common reason a filter ends up switched to unfiltered permanently. Look at the stated flow rate, and prefer a unit with a straightforward diverter so unfiltered water is genuinely convenient. That habit is also what makes the cartridge last.
Four: will it fit your tap at all
Check before anything else if you can. Unscrew the aerator with a cloth around the spout and look for a male thread outside or a female thread inside, both of which standard adapters cover. Pull out sprays, sculpted spouts and many designer taps have no usable thread and rule the format out entirely, in which case a countertop or under sink option is the alternative. Our faucet filter guide compares certified models and lists what each has actually been tested to reduce.
Questions people ask about water faucet filters
What should I compare first?
Certification, since it names what the model has actually been tested to reduce. Everything else is secondary to whether it addresses your concern.
How long should a cartridge last?
Divide the rated volume by your realistic drinking and cooking use. Filtering water for washing up shortens it dramatically.
Why does the filtered flow feel so slow?
Small cartridges restrict flow by design and more so as they load. A unit with a convenient diverter lets you use unfiltered water for everything else.
What if my tap has no aerator thread?
The format will not fit. A countertop unit on a diverter or an under sink cartridge on the cold feed are the alternatives.