Water filters and softeners: what they do, and how to check them
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Part of our filters and softeners guide
A jug on the worktop, a salt tank under the sink, a salesperson at the door. Which do you need, and how would you know if yours works? A 20-in-1 test strip at your own tap is one way to check.
Short answer: Filters take things out of tap water, mostly for taste, while softeners swap the calcium and magnesium behind limescale for sodium. The Drinking Water Inspectorate says tap water in England and Wales needs no extra home treatment as a health measure, and England's 2025 compliance was 99.97%. What a device does at your tap can be measured.
By Andrew Tomlinson, WaterKit · Last reviewed 29 October 2026
What do water filters and softeners do?
A filter takes things out of the water as it passes through, and a softener swaps the minerals behind limescale for something else. The Drinking Water Inspectorate (DWI) lists the claims sellers commonly make, from removing pesticides and nitrates to improving taste, colour and hardness, and says such claims sometimes make people worry about their tap water. The table shows the main kinds as the DWI describes them.
| Kind | What the DWI says it does | The catch |
|---|---|---|
| Activated carbon filter | Removing taste and odour is the most common reason to buy a filter, and a filter bought for that should contain activated carbon | A cartridge not changed on time stops working and may leach other chemicals or bacteria |
| Jug with ion-exchange resin beads | Some carbon jugs also hold resin that alters "temporary" hardness, so less scum on tea and possibly less kettle scale | Permanent hardness is not altered |
| Plumbed-in filter or unit | Connected directly to the supply, unlike a stand-alone jug | It must meet the Water Fittings Regulations, and the DWI says to use a qualified plumber |
| Reverse osmosis (RO) | Removes most dissolved solids and softens the water | It wastes 20 to 30% of the incoming flow, and the DWI says RO water is not generally considered suitable for drinking and cooking |
| Ion-exchange softener | Replaces calcium and magnesium with sodium, cutting scale in hot water systems | The DWI advises keeping an unsoftened kitchen tap. It needs salt and upkeep |
| Magnetic conditioner | Sold to reduce scale | The DWI says these devices do not soften the water and suggests buying only on a sale or return basis |
Our guides to filtering tap water at home, reverse osmosis and how a softener works cover each kind.
Do you need a filter or a softener at all?
Not as a health measure, according to the DWI. It says water companies must supply wholesome water suitable for drinking, cooking and washing, and that there is no need to install extra treatment at home for health protection. Its Drinking Water 2025 report puts overall compliance for England at 99.97%, across 24 water companies supplying 59.74 million consumers. Where a raw source carries traces of pesticides, the DWI says companies treat the water to bring them below the standards.
That leaves taste and scale. The DWI's hardness page calls a softener a matter of personal choice, with benefits for appliances and lathering in a hard water area. Before buying either, it advises asking your water company for its free drinking water quality report. Our guides on whether you need a filter or a softener and whether to filter your tap water take the decision step by step. WaterKit sells neither.
Which problem does each one fit?
Match the device to what you notice, because each does one or two jobs.
| What you notice | What the DWI points to |
|---|---|
| A chlorine taste or smell | A covered jug of tap water in the fridge reduces it, with unused water thrown away after 24 hours. You could also try a carbon jug filter. See why tap water can smell of chlorine |
| Limescale on the kettle and tide marks | A resin jug lowers temporary hardness only, and a softener reduces scale in hot water systems. See hard water explained |
| Brown water or particles | A filter may improve the look as a temporary measure if the fault is in your own pipes, but contact your water company first and use a plumber to find the cause. See why water turns brown or cloudy |
| A well, borehole or spring | For private supplies the DWI's point of use guidance advises whole supply treatment in preference wherever possible, and says point-of-use suitability is decided case by case on risk. See well and borehole water |
Why might your tap not match the area figure?
The area figure is an average. Your tap sits at the end of pipes and fittings that figure does not describe, and a figure for the area does not tell you what comes out of your own tap. The DWI points to several ways the difference can show up.
- Distance and plumbing. The DWI's chlorine page says a property near the treatment works may have a little more chlorine than one several miles away. It adds that a TCP-like taste can come from appliance hoses, some plastic kettles or anti-splash devices in taps, which points to home plumbing.
- Standing water and storage. The DWI says water left in your pipes for hours should be drawn off before drinking, and that bathroom and hot taps usually run from a loft storage tank, so the water is not as fresh as direct from the mains. See what the loft tank does.
- Hardness. The DWI says hardness varies with local geology, that its map shows only typical values and that areas have local variation. For an accurate figure it says to ask your water company. See how hard your water is.
- Pipes after the company's responsibility. In England in 2025, the DWI says, 447 of all compliance failures were related to consumer distribution systems.
- Private supplies. The 99.97% figure covers public supplies. The DWI's 2025 key facts list 34,102 private supplies in England, serving 818,213 people.
“An area figure is only an average, and your tap sits at the end of your own pipes, fittings and perhaps a loft tank that the figure never describes. Before you spend money on a filter or softener, I would check the water at your own tap rather than rely on the area figure alone.”
Even in one house, taps can differ. A hardness pad read at the kitchen tap and again at another tap shows whether they do.
What do the rules say about fitting one?
In England and Wales, anything you plumb in must meet the Water Fittings Regulations, and some units need your water company told first. The DWI says any product must adhere to regulation 4 of the 1999 Regulations and that suppliers must be able to show compliance, for example through UK certification.
Regulation 5 requires notice to the water undertaker, and its consent, before anyone installs a unit that incorporates reverse osmosis, or a water treatment unit that produces waste water or needs water for regeneration or cleaning. By our reading that covers a salt-regenerated softener. The company has ten working days to refuse or set conditions, and silence counts as consent. Both regulations are marked as applying in England and Wales, and we have not checked Scotland or Northern Ireland. The DWI also warns against door-to-door sellers, particularly any who offer to test your water to show it is contaminated. Prices are in our cost guide.
How do you look after a filter or softener, and can you drink softened water?
Follow the maker's instructions, according to the DWI. It says skipping the advice on cleaning and changing cartridges causes a build-up of deposits, creates a breeding ground for bacteria and lets removed material leach back into the water. For softeners it says regular backwashing and salt refills can be very important, an unoccupied property needs maintenance on your return, and a service agreement is worth considering. Intervals are in how often to change a water filter.
On drinking, the DWI advises keeping an unsoftened supply to the kitchen tap for drinking and cooking. Its reason is sodium, which has a standard in the drinking water regulations that a softener may risk breaching. It also says artificially softened water should not be used for baby feeds. See drinking softened water.
How can you check what your filter or softener is doing at your own tap?
Test the water before and after it passes through the device, and compare the pads. Fill a clean glass from the cold kitchen tap, dip one strip, wait 30 to 60 seconds and match the pads to the chart, or enter the colours in our free results checker. Then test the filtered or softened water straight away with a fresh strip. The table is WaterKit's own reading of the chart steps, not a DWI method.
| Pad | Chart steps (mg/L) | What before and after can show |
|---|---|---|
| Free and total chlorine | 0, 0.5, 1, 3, 5, 10, 20 | The DWI says companies typically keep chlorine to 0.5 mg/l or less, so your tap may already read 0 or the first step. A tap reading of 0.5 or 1 and a filtered reading of 0 indicates the filter is taking it below the chart's first step |
| Hardness | 0, 25, 50, 100, 250, 425 | On a hard supply, a softened tap reading well below the kitchen tap indicates the softener is working at that tap. A resin jug may move it one step if the drop is big enough. The steps are wide, the pad cannot tell temporary from permanent hardness, and we have not checked that its unit matches the DWI's calcium carbonate bands, so any comparison is our reading |
The strip has limits. The lead and fluoride pads first change colour at 20 mg/L and 4 mg/L, far above the 10 micrograms per litre (0.01 mg/l) and 1.5 mg/l values for consumers' taps in England under the 2016 Water Quality Regulations, so a clear pad says nothing about whether a device removes either. For lead, see our lead pipes guide. We do not claim the strip shows sodium, so it cannot tell you how much a softener has added. A change smaller than one step will not show, and a strip result is not a legal sample. The optional rapid coliform test is a presence screen on one sample, read after 24 to 48 hours, and does not replace changing cartridges on time.
The area figure is an average, and the DWI says a property near the treatment works may have a little more chlorine than one several miles away, while hoses, taps and loft tanks at your own address sit outside that figure, so your tap may differ. The pads that matter for a filter or softener are free and total chlorine and hardness. A chlorine reading that falls to 0 after a carbon filter, or a hardness reading that drops several steps at a softened tap, would indicate the device is doing its job at that tap, while a reading that does not move would suggest it is worth a closer look. Best pack: 20 strips, enough to test the kitchen tap and the filtered or softened water side by side now, and again as the cartridge or salt ages, £5.99 (or £10.99 with a bacteria test), with free 2nd Class delivery on every order. See the kit
Your water filter and softener questions
What is a water softener?
A home unit that, in the common ion-exchange type, replaces calcium and magnesium with sodium, says the DWI. See how a softener works.
What does a water filter do?
It depends on what is inside. The DWI says taste and odour is the most common reason to buy one, and that a filter bought for taste should contain activated carbon.
How does a water filter work?
The DWI sources we read name three types. Activated carbon is for taste and odour, ion-exchange resin in some jugs alters temporary hardness, and reverse osmosis removes most dissolved solids. We found no further mechanism detail on those pages.
What is reverse osmosis?
The DWI describes RO as removing most dissolved solids and softening the water, with a waste stream of 20 to 30% of the incoming flow. It says to consider it only for specialist uses. See our reverse osmosis guide.
Is filtered water good for you?
In the DWI pages we read, none says filtered water is better for health. The DWI says tap water in England and Wales needs no extra home treatment as a health measure.
Is tap water filtered?
It is treated before it reaches you. The DWI says companies must disinfect it and hold a chlorine residual in the pipes, and treat it to bring pesticides below the standards where needed. We found no filtration detail on those pages. Filters in your home are separate and optional.
How to choose a water filter?
Start with the problem. The DWI says to choose carbon for taste, buy from a reputable supplier and check the product meets the Water Fittings Regulations.
Are water filter jugs worth it?
The DWI says a covered jug of tap water in the fridge is a more cost-effective fix for taste than a carbon filter, and that a carbon cartridge must be changed on time. See do water filter jugs work?
What is the best water filter?
We do not recommend products, and WaterKit sells no filters. The DWI suggests getting your water quality report first.
Everything in our filters and softeners guide
- Do I need a water filter or a water softener?
- Should I filter my tap water in the UK?
- How do you filter tap water at home?
- How often to change a water filter
- What is reverse osmosis water?
- How does a water softener work?
- Can you drink water from a water softener?
- How much does a water softener cost, and is it worth it?
- How often should a water softener be serviced?
- Do water filter jugs work?
- What does reverse osmosis remove?
- Does a water filter remove limescale?
- Do shower filters work on hard water?
- Do magnetic water softeners work?
Sources
- Drinking Water Inspectorate, Domestic water filters and softeners (undated page; common claims, no need for home treatment as a health measure, pesticides, colour and particles, taste and odour, carbon filters, reverse osmosis, softeners and sodium, baby feeds, free water quality report, door-to-door sellers, Water Fittings Regulations, qualified plumber, maintenance, bacteria, service agreement; read 29 October 2026).
- Drinking Water Inspectorate, Water hardness (undated page; resin jugs and temporary hardness, magnetic devices, ion exchange, unsoftened drinking outlet, personal choice, map and local variation; read 29 October 2026) and Chlorine (undated page; typical residual 0.5 mg/l or less, distance from the works, fridge jug and 24 hours, carbon jug filter, standing water, loft tank, hoses and TCP-like tastes, disinfection requirement; read 29 October 2026).
- Drinking Water Inspectorate, Point of use treatment (undated page; guidance for private water supplies, aimed mainly at local authorities; whole supply treatment preferred, case-by-case risk; read 29 October 2026).
- Drinking Water Inspectorate, Drinking Water 2025: compliance with water quality standards (99.97% overall; 447 failures related to consumer distribution systems; read 29 October 2026) and Water supplies and testing (24 companies, 59.74 million consumers, 34,102 private supplies serving 818,213 people; read 29 October 2026).
- The Water Supply (Water Fittings) Regulations 1999, regulation 4 and regulation 5 (notice and consent, table items 4(e) and 4(f), ten working days; marked England and Wales; read 29 October 2026). The Water Supply (Water Quality) Regulations 2016, Schedule 1 (fluoride 1.5 mg/l and lead 10 micrograms per litre at consumers' taps; marked England and Wales; read 29 October 2026).
- WaterKit, Water Test Kit (20-in-1 strip, 30 to 60 second read, optional rapid coliform test read after 24 to 48 hours, from £3.99, free 2nd Class delivery on every order, 30-day money-back guarantee) and results portal. The chart-step table is WaterKit's own reading of the colour chart. WaterKit does not sell filters or softeners.
Contains public sector information licensed under the Open Government Licence v3.0.