How does a water softener work?

Part of our filters and softeners guide

A box under the stairs, a bag of salt beside it and a promise of no more limescale. What is it actually doing to your water?

By Andrew Tomlinson, WaterKit · Last reviewed 29 October 2026

How does a water softener work?

By ion exchange, which the Drinking Water Inspectorate says replaces calcium and magnesium (hardness) with sodium. It calls the softener the most common home water treatment system. You can see the effect at your own tap with a 20-in-1 test strip, and compare its hardness pad on a softened tap and an unsoftened one.

The DWI lists the benefits as less scale in hot water systems, and better lather and efficiency in washing machines and dishwashers. Its hardness page calls fitting one a matter of personal choice. It says that if you live in a hard water area, a softener will make lathering easier and reduce tide marks on sanitary ware. It also says the Drinking Water Directive and the UK regulations set no standard for hardness. Our limescale guide covers the scale itself.

The DWI sorts hardness into bands, in mg/l as calcium carbonate.

The third column is our own comparison. The unit basis of the pad is unconfirmed, so treat it as a rough guide.

What does a softener need to keep working?

Salt, regular backwashing and a service. The DWI says that for units like softeners it can be very important to backwash them regularly and replenish the salt. It advises following the maker's cleaning and maintenance instructions, and considering a service agreement. It says that neglect lets deposits build up, creates a breeding ground for bacteria and lets removed material leach back into the water. If the property is empty for a while, the DWI says the unit will need maintenance on your return.

In the sources we read, the DWI does not describe the regeneration step or the type of salt, so your maker's manual is the place to look. Our servicing guide goes into upkeep, and costs are in the softener cost guide.

Where does a softener go, and does it affect the boiler?

In the sources we read, the DWI does not say where most softeners are fitted, but it is clear that one tap should stay unsoftened. The DWI says that when water is softened by base exchange it is important to provide an unsoftened outlet for drinking. It gives two reasons. Too much sodium can be a problem for premature babies and for people on a low sodium diet, and the DWI says softened water should not be used to make up baby feeds. It also says the drinking water regulations include a standard for sodium, and a softener may risk breaching it. Our guide to drinking softened water covers this. The DWI also advises buying from a reputable supplier, having the unit fitted by a qualified plumber, and checking with your water company that it meets the water fittings regulations.

On the boiler, the DWI's hardness page and its manual for small supplies both say that fitting a softener just before the hot water tank or boiler is a more economical way to prevent limescale than softening the whole supply. In the sources we read, no UK official says a softener damages a boiler, but check the boiler maker's manual for any condition.

Why might your tap not match the area's hardness figure?

The area figure is an average. Your tap may differ, because the DWI says hardness varies with local geology. It says its map shows only typical hardness for England and Wales, and for an accurate figure you should ask your water company, since some areas have local variation the map does not show. A 20-in-1 strip gives you a reading at the tap itself. Our postcode hardness guide explains where to look.

A home with a softener adds a second difference. A softened tap and the kitchen tap are meant to deliver different water, so a single reading says little unless you know which tap you tested. The DWI also says artificially softened water may be aggressive to plumbing, causing leaching of copper and lead. It says soft water of 100 mg/l or less is more corrosive to pipework. The strip cannot read lead at the UK standard of 10µg/l given in the DWI manual, so it cannot show whether that is happening in your pipes. Our guide on how to tell if your house has lead pipes covers that.

“I would treat the area figure as a starting point and not the final word on your own tap. Local geology varies, and with a softener in the house it also matters which tap you test, so I would check the kitchen tap and a softened one side by side.”

What one reader found

A Mumsnet reply to a thread asking whether a softener is a good investment describes one owned for about 10 years. The poster wrote "we live in SE England so very hard water". They report no limescale in the home and say they do not go through much salt, though they cannot remember what it cost. It is one household's experience, not a measurement. Shared on Mumsnet.

What can a test strip show about a softener?

It indicates the hardness of the water at the tap you test. The hardness pad has colour steps at 0, 25, 50, 100, 250 and 425 mg/l (the unit basis of the pad is unconfirmed). Fill a clean glass or cup, dip one strip, wait 30 to 60 seconds and match the pad to the chart, or enter the colours in our free results checker. Test the kitchen tap and a softened tap side by side, and test again after a service. The copper pad (steps 0, 0.2, 0.5, 1, 2 and 5 mg/l) is worth reading too, since the DWI says softened water may cause leaching of copper. Our hardness testing guide has the steps.

Three limits are worth knowing, and the first two are our reading of the chart.

• A softened tap that reads at the 50 or 100 step sits within the DWI's soft band of up to 100 mg/l. The strip cannot tell you why it reads there, so the maker's manual is the place to check the settings.
• The strip does not show how full the salt tank is, whether the unit has just regenerated, or the sodium level.
• A strip result is not a legal sample.

The area hardness figure is an average, and the DWI says local geology means your own tap may differ, so a reading at your tap is worth having. The hardness pad is the one that matters most here. A step that moves after a service, or differs between the kitchen tap and a softened tap, would indicate the unit is treating those taps differently. The copper pad matters too, because the DWI says softened water may cause copper to leach, so a higher copper step on the softened tap would indicate copper reaching the water at that tap. Best pack: 20 strips, enough to test the kitchen tap and a softened tap now and again after a service, £5.99 (or £10.99 with a bacteria test), with free 2nd Class delivery on every order. See the kit

Your water softener questions

What is a water softener?

A unit that removes hardness from the water supply by ion exchange. The DWI calls it the most common home water treatment system.

What does a water softener do?

It lowers hardness. The DWI lists less scale in hot water systems and better lather and appliance efficiency as the benefits. It adds that artificially softened water may be aggressive to plumbing. See do I need a water filter or a water softener?

How does a water softener work with salt?

The DWI says most softeners replace hardness with sodium, and that it can be very important to replenish the salt. In the sources we read, it does not describe the regeneration step, so check your manual.

What is a duplex water softener?

We found no definition in the sources we read. Ask the maker how its unit supplies softened water while it regenerates.

What is water softener salt?

In the sources we read, the DWI says to replenish salt but does not define the type, so use whatever the maker's manual specifies.

Can you use granular salt in a water softener?

We found no guidance on granular salt in the sources we read, and your manual is the place to look.

Can I use water softener salt in my dishwasher?

We found no source that answers this. Check the salt bag and your dishwasher's manual. Our dishwasher salt guide covers the dishwasher side.

What size water softener do I need?

We found no sizing guide in the sources we read. The DWI says to ask your water company for an accurate hardness figure, and the maker can size a unit from that.

Will a water softener damage my boiler?

In the sources we read, no UK official says so. The DWI describes fitting a softener before the hot water tank or boiler as a way to prevent limescale. Your boiler maker's manual may set conditions.

Does a water softener reduce water pressure?

We found no figure for a home softener in the sources we read. The DWI notes a substantial pressure reduction through a reverse osmosis unit, which is a different device. Ask the softener maker for the figure at your flow rate.

How does a commercial water softener work?

We found no separate guidance on commercial units in the sources we read. The DWI describes two softening methods, lime-soda softening, where added lime and sodium carbonate make the hardness compounds precipitate, and ion exchange, which it says is common in domestic softeners.

Sources

1. Drinking Water Inspectorate, Domestic water filters and softeners (undated page; ion exchange, most common home treatment, unsoftened kitchen tap, sodium standard, baby feeds, backwashing and salt, service agreement, neglected units, RO pressure; read 29 October 2026).
2. Drinking Water Inspectorate, Water Hardness / Hard Water (undated page; hardness bands, no hardness standard, scaling, temporary hardness, personal choice, plumbing leaching, local variation and the map; read 29 October 2026).
3. Drinking Water Inspectorate, Manual on Treatment for Small Water Supply Systems (undated page, adapted from a March 2001 report on private supplies; softener before the hot water tank or boiler; read 29 October 2026).
4. Mumsnet, Water softener for your home. Good investment? (thread of 7 June 2024; one poster's experience, not a measurement; read 29 October 2026).
5. 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) and results portal. The strip-chart column and the comments on expected readings are WaterKit's own reading of the colour chart steps.

Contains public sector information licensed under the Open Government Licence v3.0.