What is the best type of under-sink water filter for UK hard water?

For limescale and taste in a hard water region, a carbon block or dedicated limescale filter handles the everyday complaints — kettle scale, chlorine smell, flat-tasting tea. If you also want dissolved minerals, nitrates and PFAS reduced, reverse osmosis is the only under-sink technology that does it, at the cost of cupboard space and some waste water.

Does an under-sink filter soften water?

No. Softening removes calcium and magnesium from the whole house and needs a plumbed-in softener with salt. An under-sink filter treats drinking water at one tap. Reverse osmosis removes most of those minerals from the water it filters, but it does not protect your boiler, shower or washing machine.

How much does an under-sink water filter cost to run in the UK?

Cartridges are the running cost. A carbon block cartridge typically lasts 6-12 months; an RO membrane 2-3 years with pre-filters changed annually. Budget for one or two cartridge changes a year rather than a one-off purchase.

Will an under-sink filter fit my kitchen?

Most single-cartridge filters need roughly 40cm of clear height beside the waste pipe. Tank-based RO systems need that plus room for the storage tank, so a compact or tankless system is the sensible pick in a flat or a sink base packed with a bin and a boiler.

Do I need a separate tap?

For reverse osmosis, yes — filtered water comes out of its own small tap. Inline carbon and limescale filters can be plumbed into the cold feed of an existing tap, or paired with a 3-way tap so you get filtered water alongside hot and cold.

Buying guide

Best under-sink water filter UK: how to choose

This is a guide to our own range, not a roundup of other retailers' products. We only sell under-sink filtration, so rather than pretend to rank the whole market we have set out what each type genuinely fixes, what it does not, and which of our five systems suits which kitchen. If the honest answer is that you need a whole-house softener instead, we say so.

The short answer

Taste and smell only: a carbon block filter. Cheapest to buy and run.
Kettle scale in a hard water area: a dedicated limescale filter.
Widest contaminant removal: reverse osmosis — compact, tank or tankless depending on space.
Protecting the whole house: not this product category at all — you want a water softener.

Four things that decide the right filter

Start with the symptom

Scale in the kettle points to hardness. A swimming-pool smell points to chlorine. Cloudy or gritty water points to sediment. Buying by symptom stops you paying for filtration you do not need.

Measure the cupboard first

The most common return reason in this category is a system that does not fit. Measure clear height and depth beside the waste pipe before you choose between compact, tank and tankless.

Price the cartridges, not just the unit

A cheap unit with expensive proprietary cartridges costs more over three years. Check cartridge price and interval before you commit to a system.

Check your mains pressure

Tank-based RO relies on mains pressure to push water through the membrane. Low pressure means slow filling. Tankless systems pump, so they are less sensitive.

Which Clarisink filter fits which kitchen

Best for taste, smell and general filtration

Carbon Block Filter

A single carbon block cartridge is the least intrusive way to fix chlorine taste and odour. Nothing to power, nothing to drain, and it fits in a cupboard already crowded with a bin and a boiler.

Worth knowing: Carbon does not remove dissolved solids. If your goal is a lower TDS reading, this will not deliver it — reverse osmosis will.

View the Carbon Block Filter

Best for kettle limescale in a hard water area

Limescale Defender

Aimed squarely at scale-forming minerals in the South East and Midlands. Choose this over RO if your complaint is furring and chalky residue rather than the full spectrum of contaminants.

Worth knowing: This is scale inhibition, not softening. The polyphosphate stops calcium and magnesium from crystallising and sticking to surfaces — it does not take them out of the water. A hardness test will read the same before and after.

View the Limescale Defender

Best for small kitchens and flats

RO-300

The most compact reverse osmosis option. Full RO filtration where a tank-based system simply will not physically fit.

Worth knowing: Three stages rather than four means one less carbon stage, so chlorine taste removal is good but not quite as thorough as the RO-400.

View the RO-300

Best all-round reverse osmosis system

RO-400

Tank storage means a normal flow rate at the tap and no electricity. The sensible default for a family kitchen with standard cupboard space.

Worth knowing: RO sends water to the drain as part of how it works — expect roughly two litres wasted per litre filtered. If your household is metered, factor that into running cost.

View the RO-400

Best for busy households and no stored water

RO-500

Tankless, filters on demand, better pure-to-drain ratio and a live TDS readout. Needs a socket, and it costs more.

Worth knowing: Needs a 13amp socket under the sink. If you have no power there, you need the RO-400 instead — this is the single most common reason a tankless unit has to go back.

View the RO-500

Common questions

What is the best type of under-sink water filter for UK hard water?: For limescale and taste in a hard water region, a carbon block or dedicated limescale filter handles the everyday complaints — kettle scale, chlorine smell, flat-tasting tea. If you also want dissolved minerals, nitrates and PFAS reduced, reverse osmosis is the only under-sink technology that does it, at the cost of cupboard space and some waste water.
Does an under-sink filter soften water?: No. Softening removes calcium and magnesium from the whole house and needs a plumbed-in softener with salt. An under-sink filter treats drinking water at one tap. Reverse osmosis removes most of those minerals from the water it filters, but it does not protect your boiler, shower or washing machine.
How much does an under-sink water filter cost to run in the UK?: Cartridges are the running cost. A carbon block cartridge typically lasts 6-12 months; an RO membrane 2-3 years with pre-filters changed annually. Budget for one or two cartridge changes a year rather than a one-off purchase.
Will an under-sink filter fit my kitchen?: Most single-cartridge filters need roughly 40cm of clear height beside the waste pipe. Tank-based RO systems need that plus room for the storage tank, so a compact or tankless system is the sensible pick in a flat or a sink base packed with a bin and a boiler.
Do I need a separate tap?: For reverse osmosis, yes — filtered water comes out of its own small tap. Inline carbon and limescale filters can be plumbed into the cold feed of an existing tap, or paired with a 3-way tap so you get filtered water alongside hot and cold.