If you live anywhere with hard water, a softener quietly does a lot of work in the background. It only asks one thing of you in return, and that is the right salt in the tank. This article explains the three types you will see for sale, how to tell which one your machine takes, why purity matters more than most people realise, and what it costs to keep a household topped up through the year. It also covers the two problems that catch people out, a hard crust across the top of the tank and a sludge at the bottom, and how to avoid both.
Why the type of salt matters at all
A water softener works by passing hard water through a bed of resin beads that hold onto the calcium and magnesium responsible for limescale. The beads fill up eventually, and the machine then rinses itself with a strong salt solution to flush those minerals away and start again. That rinse is the only reason salt is involved. Everything else the softener does is automatic.
So the salt is not really a consumable in the way a filter is. It is the ingredient the machine makes its own cleaning fluid from. If the salt is the wrong shape for the tank, or if it carries a lot of insoluble material, the machine cannot make that solution properly and it starts to underperform long before anything actually breaks. That is why suppliers such as Salt Giant sell it by type and by purity rather than simply by weight, and why the cheapest bag on a shelf is not automatically the sensible buy.
The three types you will come across
There are only three formats in common use in British homes, and they are easy to tell apart on sight.
Tablet salt
Small, compressed white tablets, roughly the size of a coin. This is the default for most modern softeners with a loose salt tank, and it is what the majority of households end up buying. The tablets are dense enough to dissolve at a steady rate rather than collapsing all at once, which is exactly what the machine wants. It comes in ten kilogram and twenty five kilogram bags, so you can choose between something you can carry comfortably and something that works out cheaper per kilo.
Granular salt
Loose crystals, a bit like very coarse cooking salt. Granular tends to be specified by older units, higher capacity machines and some commercial installations. It dissolves faster than tablets because there is more surface area, which suits systems designed around it and can cause problems in systems that are not. The same granular product is often sold for dishwashers and for saltwater pools, so it is worth checking the bag if you have more than one use in the house.
Block salt
Solid rectangular blocks, usually four kilograms each and sold in pairs. These are made for compact cabinet softeners that sit under a kitchen sink and take two blocks side by side. Block salt is popular in the United Kingdom because several of the best selling softeners here are built around it. If your machine has two neat recesses rather than an open tank, blocks are what it needs and nothing else will fit.
The table below sets the three side by side.
| Type | What it looks like | Usually suits |
|---|---|---|
| Tablet | Compressed white tablets the size of a coin | Most modern softeners with an open salt tank |
| Granular | Loose, free flowing crystals | Older, larger and some commercial units |
| Block | Solid blocks, normally four kilograms, sold in pairs | Compact cabinet softeners that take two blocks |
Working out which one you need
The quickest answer is to lift the lid. If there are two shaped recesses, you need blocks. If there is an open tank with loose salt in it, you need tablets or granular, and the manual will tell you which. Most people find the manual has long since disappeared, in which case the make and model number is usually printed on the front or inside the lid, and the manufacturer will list the correct salt online.
If you are still not certain, tablets are the safer guess for an open tank softener bought in the last fifteen years or so. What you should not do is switch types on a whim. A machine designed for tablets will often cope with granular for a while and then start behaving oddly, which is a confusing fault to diagnose because nothing has visibly gone wrong. Good suppliers of water softener salt will list which format goes with which kind of system, and it takes a minute to check before you order a pallet of the wrong thing.
Purity is the part people skip
Softener salt is usually well over ninety nine per cent sodium chloride, and the small remainder is what separates a good bag from a poor one. Anything that does not dissolve has to go somewhere, and where it goes is the bottom of your brine tank. Over a few years that builds into a grey sludge that has to be scooped out by hand, and in the meantime it interferes with the machine drawing brine cleanly.
This is also why table salt, cooking salt and ordinary sea salt are a genuinely bad idea, whatever the internet says about saving a few pounds. They are far too fine, they often contain anti caking agents and iodine, and they dissolve into a paste that can clog the valve and damage the resin bed. Replacing a resin bed costs a great deal more than a year of proper salt.
The two problems worth knowing about
The first is bridging. A hard crust forms across the top of the salt and leaves an empty gap underneath, so the tank looks full while the water below has nothing to dissolve. The softener carries on regenerating with plain water and you gradually notice scale creeping back. Damp air in a garage or utility room makes it more likely, as does letting salt sit for months without being disturbed. If you tap the surface with a broom handle and it feels solid rather than loose, break it up gently and let it settle.
The second is mushing, which is the opposite problem. Salt that dissolves too quickly for the system recrystallises into a thick slush at the bottom of the tank and blocks the flow. It is nearly always caused by the wrong format or by very low quality salt, and it is the more awkward of the two to clear because the tank has to be emptied.
Both are avoidable with a monthly glance under the lid. Aim to keep the tank at least half full and a few inches of salt above the water line, and top it up before it runs low rather than after.
What it costs to run over a year
A typical family home uses somewhere in the region of two to five bags of salt a year, depending on how hard the local water is and how many people are running showers and washing machines. Larger households in very hard water areas will be at the upper end of that, and a couple in a softer area may use less. At normal bag prices that puts the running cost roughly in the range of forty to a hundred pounds a year, which is modest set against a new boiler or a replacement dishwasher.
Buying in bulk brings the price per kilogram down considerably, and a half pallet delivered to the door is usually far cheaper than picking up single bags as and when. The trade off is storage. Salt needs to stay dry and off a bare concrete floor, because concrete draws moisture up into the bottom bags and sets them solid. A wooden pallet, a couple of planks or a shelf in the garage solves it. Keep the bags closed until you use them and they will keep indefinitely.
The short version
Check the lid, match the format, buy decent quality, and look in the tank once a month. That really is the whole job. A softener that gets the right salt will run for years with almost no attention, and the difference shows up in the small things: a kettle that stays clean inside, taps that wipe dry without a chalky film, and appliances that last the length of time they were meant to.
