Pool salt is a purity specification, not a brand

Why the bag matters at all

A salt chlorine generator does not consume salt. It borrows it: the cell splits sodium chloride into chlorine, the chlorine sanitizes and then reverts to chloride, and the cycle repeats. You top up only for what leaves by splash-out, backwashing and rain overflow.

So the salt you add stays in the water for the whole season, and anything else in the bag stays with it. That is the entire argument for buying salt on its purity specification: the impurities do not get used up, and the cell is the expensive component they act on. Our salt systems guide covers how the cell works and what shortens its life.

What the generator makers actually require

This is not a question anyone needs to guess about, because three major manufacturers publish it. Pentair’s IntelliChlor guide asks for salt that is at least 99.8% pure NaCl, preferably evaporated, granulated, food quality and non-iodized with no additives, and warns specifically against anti-caking agents containing sodium ferrocyanide, against calcium or potassium chloride, and against rock salt.

Hayward’s Aqua Rite manual asks for greater than 99% pure sodium chloride and says plainly: do not use rock salt, salt with yellow prussiate of soda, salt with anti-caking additives, or iodized salt. Intex asks for at least 99.8% pure and warns against iodized and yellow-tinted salt.

Read against those requirements, the four bags here sort themselves. AQUASALT publishes 99.9% typical purity and an evaporated production process. Morton’s Professionals’ Choice data sheet publishes 99.00% typical — below what Pentair and Intex ask for, although it does state no additives and NSF certification. Morton Pool Salt’s safety data sheet says over 99% with no additives. Aqua Joe publishes no purity figure at all.

How much salt, and how to work it out

Two of the bags here publish the number that makes this arithmetic possible: one 40 lb bag raises salinity by about 480 ppm in 10,000 gallons. From that, and your target, the initial charge falls out.

Three practical rules follow from that. Add salt with the pump running and brush it around rather than dumping it in one place. Give it time to dissolve fully before trusting a cell reading — Pentair notes its own salt sensor is accurate to within 500 ppm. And always aim slightly low: too little salt is a top-up, too much is a partial drain.

The targets are not the same, and that matters

The three manufacturers we checked publish genuinely different targets. Hayward’s ideal band is 2,700 to 3,400 ppm with 3,200 optimal. Intex publishes 3,000 ppm ideal within an operating range of 2,500 to 3,500. Pentair recommends 3,600 ppm for optimum water condition, turning off below 2,600 and warning about corrosion above 4,500.

Pentair’s recommended level sits above Hayward’s entire ideal band and above Intex’s operating ceiling. There is no universal correct salinity; there is only the figure in your own manual. Anyone quoting a single number for all salt pools is repeating folklore.

It is also worth knowing that salt becomes noticeable to taste at around 3,500 to 4,000 ppm per Hayward, which is well below seawater and part of why a salt pool feels softer rather than briny. Our salt versus chlorine comparison deals with what a salt pool actually is, which is a chlorine pool that makes its own chlorine.

Which bag to buy

If your manual specifies 99.8%: AQUASALT is the only bag here that documents it, at 99.9% typical with an evaporated process. That is the defensible choice for a Pentair or Intex system.

If you want the arithmetic printed on the bag: Morton’s Professionals’ Choice, which publishes both the ppm-per-bag figure and a full data sheet, and carries NSF certification. Note its published 99.00% typical purity against the 99.8% some manuals ask for.

If you need salt today: Morton Pool Salt, from the hardware store, with a safety data sheet confirming no additives. Check the bag says pool.

What we would avoid: any bag that publishes no purity figure at all. Not because it is necessarily bad salt, but because purity is the one specification that protects the cell, and a product that does not state it cannot be checked against your manual.

How we picked

We have not tested these salts. Scores are our reading of published specifications, weighted toward the makers who publish purity, additives and production method — the three things generator manuals actually specify. Where a retailer’s purity claim contradicts the manufacturer’s own data sheet, as it does for one product here, we have used the manufacturer’s figure and said so.

Why purity protects the part that costs money

A salt cell works by passing current through the water across titanium plates. Anything in the bag that is not sodium chloride ends up on or around those plates, and the plates are the expensive component — on most systems, replacing a cell costs a meaningful fraction of replacing the whole generator.

Pentair names the specific culprit rather than talking in generalities: anti-caking agents containing sodium ferrocyanide, also known as yellow prussiate of soda, which it warns can discolor fittings and surface finishes. It also rules out calcium chloride and potassium chloride as salt sources, and rock salt, on the grounds that insoluble impurities shorten cell life. Hayward’s manual bans the same list.

That is the whole case for reading the bag. Salt is cheap; cells are not; and the difference between a compliant bag and a non-compliant one is usually a couple of dollars. Our salt systems guide covers how the cell works and what else shortens its life.

Topping up, and where the salt actually goes

Salt is not consumed by chlorination, so an established pool needs far less than people expect. What it loses is water: splash-out, backwash water, a to-waste vacuum, rain overflowing the skimmer, and a partial drain. Every gallon that leaves takes its dissolved salt with it; evaporation does not, because the salt stays behind.

That gives a simple rule. Top up after anything that removes water, not on a calendar. A season with two backwashes and a wet August may need nothing at all; a season with a flocculant treatment and a partial drain to fix cyanuric acid will need a bag or two, as our draining guide explains.

Test before adding, every time. Cell readings drift, and Pentair notes its own salt sensor is accurate to within 500 ppm — which is a sixth of the whole working range. A strip or a meter costs less than a bag of salt, and our test strip roundup covers what to buy.

Too little, too much, and what each does

Both ends of the range have published consequences, and they are not symmetrical.

Too low and the cell stops. Pentair states that a concentration below 2,600 ppm will cause the unit to turn off, which on a hot week means a pool with no sanitizer while you work out why. Low salt also makes cells work harder, which is the quiet version of the same problem.

Too high and you are into corrosion territory: Pentair warns that concentrations above 4,500 ppm may cause excessive corrosion. Hayward notes that salt starts to be tasted at about 3,500 to 4,000 ppm, which is a useful early warning that costs nothing.

The asymmetry matters because there is no way to remove salt except replacing water. Adding is easy; overshooting means diluting a pool you have just filled and rebalancing everything else with it. Aim low, test, and add again.

Salt pools over winter

The cell comes off and comes indoors with the rest of the equipment — it is a temperature-sensitive electronic component with water inside it, which is the worst combination for a freeze. Our equipment storage guide covers the wider close, and the same rule applies here: drain it, dry it, store it somewhere that does not freeze.

The salt itself stays in the pool. It does not degrade, it does not need removing, and a closed pool reopens in spring with essentially the salinity it had in autumn, minus whatever water left. That is the quiet advantage of a salt pool over tablets: the sanitizer supply does not have a shelf life, which our payback guide counts alongside the hardware cost.

What a salt pool does not change

Buying the right salt is worth doing, and it is worth being clear about what it does not buy. A salt pool is a chlorine pool that manufactures its own chlorine, as our comparison of the two explains, and every reading you tested before still needs testing now: free chlorine, pH, alkalinity and stabilizer.

Two of those get harder rather than easier. Salt cells drive pH upward as a matter of normal operation, which is the argument for the buffering covered in our borates guide. And stabilizer still matters, because the chlorine a cell makes is consumed by sunlight exactly like the chlorine from a tablet.

The salt itself is the one consumable that behaves well: no shelf life, no degradation, and a top-up only when water physically leaves the pool.