Small softeners sized for a single outlet do exist, and they work by the same ion exchange as a whole house unit in a much smaller vessel. They are a legitimate product for specific duties, and for the household complaint that usually prompts the search they are the wrong shape of answer, because hardness does its damage almost entirely somewhere else.
Where hardness actually costs you
In the water heater, where scale insulates the element or exchanger and wastes energy on every cycle. In the washing machine and dishwasher, whose heating elements fail scaled. In the hot pipework, which narrows over years. On the shower screen and in the bathroom, where the visible film and spotting appear. A softener at the kitchen sink reaches none of that, because every one of those is upstream or elsewhere, and treating one tap changes nothing about them.
What a point of use softener is genuinely for
Protecting one appliance. A small softener or a cartridge on the feed to an espresso machine, a steam oven, a combination boiler or an ice maker stops scale in that one heat exchanger, which is often the highest value thing in the house to protect and the manufacturer may require treatment for the warranty. Portable units are used for caravans and boats. In every case the target is a specific piece of equipment rather than the household's water.
The formats and their limits
Small cartridge softeners hold a little resin and are exhausted quickly on hard water, then regenerated by hand with salt or simply replaced. Polyphosphate cartridges do not soften at all: they sequester hardness so it stays in solution and deposits less, which protects an appliance without changing anything you would notice at the tap. Neither has the capacity for a household. The USGS classes water above 180 mg/L as calcium carbonate as very hard, and capacity falls sharply at that end.
What to do instead
If the complaint is scale, spotting, poor lather or stiff laundry, the answer is a whole house softener, because those are property level problems. If it is one appliance, a point of use cartridge is proportionate. If it is drinking water quality rather than hardness, a filter or reverse osmosis at the sink is the right product and softening is not involved at all. Our softener installation guide covers sizing a whole house unit.
Questions people ask about water softener for sink
Can I soften just the kitchen sink?
You can, and it rarely helps, because hardness does its damage in the water heater, appliances and bathroom rather than at that tap.
What is a point of use softener good for?
Protecting one appliance: an espresso machine, steam oven, combination boiler or ice maker, often required for the warranty.
Does a polyphosphate cartridge soften water?
No. It holds hardness in solution so it deposits less. A hardness test reads exactly the same afterwards.
What should I fit if the whole house has hard water?
A whole house softener. Scale, spotting, poor lather and stiff laundry are property level problems, not sink level ones.