Salt is the only thing a water softener ever asks of you, and it asks every few weeks for fifteen years. The format is decided by the unit, not by preference: blocks for the twin-cylinder cabinets, tablets for almost everything else, granular for the few that say so. What you can choose is where to buy it and how much to buy at once, which is where the running cost is actually decided.
According to TapWater.uk, a family home in very hard UK water typically spends £5 to £10 a month on softener salt in 2026, with tablets in 25 kg sacks the cheapest per kilo and 4 kg blocks the format twin-cylinder softeners require. The highest-rated softener salt on Amazon UK is Hydrosalt's 25 kg tablets at 4.7 stars from more than 2,000 reviews.
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Look at the cabinet. A slot the shape of a house brick takes blocks. A tank with a lid takes tablets, unless the manual says granular. Each format is pure sodium chloride; the resin does not care which, but the mechanism that dissolves it does.
The default. Buy the biggest sack you can store dry and lift safely.
Not a choice, a requirement of the softener. Convenient enough that nobody minds.
Only if the manual says so. Do not put it in a unit designed for tablets.
The workhorse. Three sizes, from a 10 kg bag you can carry up a loft ladder to a three-sack bundle that lasts a very hard water household a season. Look for BS EN 973 Class A on the bag where the maker states it.
Best-rated tablets on Amazon UK
Hydrosalt Salt Tablets 25kg
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The lighter bag, standard-marked
Monarch Ultimate Salt Tablets 10kg
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Bulk buy: three sacks in one delivery
AQUASOL Salt Tablets 3 × 25kg
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For Harvey, Kinetico and BWT block-salt cabinets. Each brand sells its own blocks; third-party 4 kg blocks are the same size and fit the same slot. You pay more per kilo than tablets and in return never touch a scoop.
For twin-cylinder cabinets
BWT Block Salt 8kg (2 × 4kg)
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Fine-grained and quick to dissolve, which is why some older softeners and every dishwasher specify it. In a modern tablet unit it can cake into a bridge over the brine tank, so only use it where the manual says to.
Only where the manual specifies it
Hydrosoft Granular Salt 10kg
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Two things set the rate: how hard the water is, because harder water exhausts the resin sooner, and how much water the household runs. A metered softener only regenerates when the resin is spent, so a couple in 200 mg/L water might top up every couple of months while a family of five in 350 mg/L water is lifting a sack every few weeks. The sensible budget for a family home in very hard water is £5 to £10 a month, and it falls if you buy in bulk.
Storage matters more than brand. Salt kept somewhere damp cakes into a solid lump that will not pour and, in the tank, forms a bridge that leaves the softener rinsing with plain water. Keep sacks sealed, off a concrete floor, and do not overfill the brine tank.
A full salt tank proves nothing if a bridge has formed or the resin is spent. A hardness test tablet dropped into a glass from the softened tap gives a yes-or-no answer in a minute. Test the hard kitchen tap alongside it and you will see the difference the unit is making, or not.
A one-minute answer
SimplexHealth Total Hardness Yes/No Test Tablets (pack of 2)
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Do not have a softener yet? Start with do I need a water softener and then best water softener UK.
The format your softener was designed for. Non-electric twin-cylinder units from Harvey and Kinetico, and BWT's block-salt models, take 4 kg block salt. Most metered single-cylinder units from Monarch, BWT and Water2Buy take salt tablets poured into the brine tank. A few older units and dishwashers specify granular salt. Whatever the format, it should be pure sodium chloride made for softeners, ideally marked BS EN 973 Class A.
No to table salt and rock salt: table salt contains anti-caking agents and iodine, rock salt carries grit and clay, and both can foul the resin and the valve. Dishwasher salt is pure sodium chloride and will not harm the resin, but it is sold in small bags at a high price per kilo and its grain size can bridge in a brine tank. Buy softener salt.
It depends on how hard the water is and how much you use: a large family in very hard water regenerates far more often than a couple in merely hard water. As a rule of thumb, a family home in very hard water budgets £5 to £10 a month, which is roughly a 25 kg sack of tablets or several pairs of blocks. A metered unit only regenerates when the resin is spent, so it uses less than a timer unit set to rinse on fixed days.
Tablets, per kilogram. They come in 10 kg and 25 kg sacks and three-sack bundles, and the price falls with size. Block salt is sold in pairs of 4 kg blocks and costs more per kilo; what you pay for is a format that drops into the cabinet without scooping or spillage and that the twin-cylinder brands require. Neither softens water better than the other.
BS EN 973 is the European standard for sodium chloride used to regenerate ion exchange softeners. Class A is the higher purity grade. It tells you the salt is free of the impurities that foul resin, which is what you want. Some reputable salt is not labelled with the standard, so its absence is not proof of a problem, but its presence is a reassurance.
Usually a salt bridge: a crust forms over the brine tank and the salt below it looks full while the water underneath has no salt to dissolve. Break it up with a broom handle. Other causes are a blocked brine line or a stuck float. If the salt level is fine and the water still feels hard, test the softened tap with a hardness tablet before calling anyone; it tells you in a minute whether the resin is doing its job.