A faucet filter attachment lives or dies on its adapters. The filter body is standard, the media is standard, and whether the whole thing works in your kitchen depends entirely on what is inside the end of your spout. Five minutes with a cloth answers it, and the answer is often no, which is worth knowing before the box is opened.
Find out what your spout has
Wrap a cloth around the end of the spout to protect the finish and unscrew the aerator counter clockwise as viewed from below. There are three outcomes. A male thread on the outside of the spout is the most common and is covered by a standard adapter. A female thread inside the spout is equally common and takes a different adapter. Or there is no thread at all, because the outlet is a cartridge insert, a pull out spray, or a sculpted spout, in which case no attachment will fit.
The adapters that come in the box
Most kits include a small set of adapters and washers covering the common male and female sizes, which is why the same product fits a wide range of taps. Keep the ones you do not use, along with the original aerator, because you will want both when the filter comes off. If a kit does not include the size you need, adapters are sold separately, but check that the manufacturer supports the combination rather than improvising with plumbing fittings, since the seal here is against household pressure at a joint you cannot see.
Fitting it without splitting anything
Fit the adapter with its washer and hand tighten only. The collar is plastic and the spout is metal, so the collar loses: over tightening cracks it, and the crack usually shows as a weep several days later rather than immediately. Run the unit for the flush time the manual states, which clears carbon fines and is why the first water looks grey. Check the joint after the first few uses. If the tap is a slender basin style and the filter is heavy, consider whether the spout will take the load.
When nothing fits
Three alternatives. A countertop unit fed by a diverter needs the same aerator thread, so it does not help if the thread is the problem. An under sink cartridge plumbed into the cold feed treats the tap without touching its outlet at all, provided the cabinet has space. Or a gravity unit on the bench, which needs no connection of any kind and works regardless of the tap. Our faucet water filter guide compares certified attachments and what each is tested to reduce.
Questions people ask about faucet filter attachment
How do I know if a faucet filter will fit my tap?
Unscrew the aerator and look. A male thread outside or a female thread inside are both standard; no thread at all means the format will not fit.
What if the kit has no adapter for my thread?
Adapters are sold separately, but use ones the manufacturer supports rather than improvising, since the joint holds household pressure.
Why did the collar crack?
Over tightening. The plastic collar is hand tight only against a metal spout, and the resulting weep often appears days later.
What are the options if nothing fits?
An under sink cartridge on the cold feed, which ignores the spout entirely, or a gravity unit on the bench that needs no connection at all.