Saltwater vs Freshwater Pools: What the Words Actually Mean

A salt water pool is not ocean water and not strictly fresh water either. At a typical 3,200 ppm it is about a tenth as salty as the sea, which the US Geological Survey would class as moderately saline. “Freshwater pool” has no technical meaning at all. People use it for an ordinary chlorine pool, and sellers use it as a label for mineral or ionizer systems.

This page is part of our salt water pools hub. It is about the words and the water itself. If you want to compare how the two kinds of pool are sanitized and what they cost, read salt water pool vs chlorine instead.

What “fresh” and “saline” mean to a hydrologist

The USGS sorts water by how much dissolved salt it carries. Its salinity scale runs like this:

A separate USGS project defines brackish water as having dissolved solids “between 1,000 and 10,000 milligrams per liter.”

Hayward’s manual asks for 2,700 to 3,400 ppm, with 3,200 ppm as optimal. One Pentair guide aims higher, 3,000 to 4,500 ppm with 3,600 ideal. Either way, a salt pool sits around the line between slightly and moderately saline. Brackish is the fair word for it. Fresh is not.

That surprises people who assume “salt pool” means something close to the beach. It is much closer to your tap than to the ocean.

Where a salt pool sits next to other water

The tear figure is our arithmetic, not a published number. Tears contain more than salt, so we compared their osmotic strength instead. EyeWiki gives normal tear osmolarity as “296±9.8 mOsm/L,” and StatPearls lists 0.9% saline, which is 9,000 ppm salt, at 308 mOsm/L. Tears land just under 0.9% saline. Pool water at 3,200 ppm works out to about 110 mOsm/L, roughly a third of that. The ocean is saltier than tears by about four times. That is why a well-run salt pool is gentler on open eyes than the sea. Our page on whether you can open your eyes in a salt water pool covers what actually makes them sting.

A chlorine pool is not salt-free either

Here is the part most comparisons miss. Orenda’s technical blog explains that “All types of chlorine, when reduced (used up), become chloride (Cl-), which is basically a salt,” and that “Liquid chlorine leaves behind sodium and chloride, which is common salt (NaCl).” A pool dosed with liquid chlorine for years slowly builds up salt of its own. How much depends on how much chlorine went in and how much water has been replaced since. None of our sources puts a number on it, so we will not guess.

The reverse is true too. A salt cell makes chlorine from the salt, the same hypochlorous acid you get from bleach. A salt pool is a chlorine pool with its own chlorine plant.

This matters when you convert a pool to salt water. An older chlorine pool may already hold some salt. The chlorinator panel will not tell you how much with any certainty, because it has no salt sensor. It reads how easily current crosses the cell and infers a number. Orenda warns that depending on the system, the reading may reflect salinity or total dissolved solids. Test with an independent salt test kit before you run the salt calculator, or you will overshoot.

“Freshwater” as a sales word

Some systems are sold as “freshwater” pools. One US copper ionization seller describes its system as keeping water clear without “high levels of traditional pool chemicals” and says it removes the need for chlorine tablets and shock.

The pool industry’s own trade group disagrees. The Pool and Hot Tub Alliance fact sheet on ionizers says: “As a stand alone device, ionizers do not sanitize pool or spa water. They must be used in conjunction with an EPA-registered sanitizer.” We trust the trade group here, because it is not selling the device. The same fact sheet notes that electrolytic ionizers must be registered as pesticides with the EPA. So when a pool is sold to you as “freshwater,” ask what the sanitizer is. There should be one.

Can you taste the salt?

Probably, faintly. The EPA’s secondary standard for chloride in drinking water is 250 mg/L, where the noticeable effect is “salty taste.” Chloride makes up about 61% of salt by weight, so a pool at 3,200 ppm salt carries about 1,940 ppm chloride. That is close to 8 times the level where the EPA expects people to notice. The ocean, at about 35,000 ppm, is roughly 11 times saltier than the pool. You notice salt in a salt pool. You do not get a mouthful of the sea.

Do you float better in a salt pool?

Barely. Dissolved salt makes water denser, and the USGS confirms that “ocean water, which contains dissolved salt, is denser than pure water.” The question is how much.

Using reference densities at 68°F, pure water is 0.9982 g/cm³ and a 1% salt solution by weight is 1.0053 g/cm³. Pool water at 3,200 ppm is about 0.32% salt, which works out to about 1.0005 g/cm³, or 0.23% denser than fresh water. Average seawater is near 1.025 g/cm³, about 2.7% denser than fresh.

In practical terms, for a 180 lb swimmer:

The salt pool gives you about a twelfth of the ocean’s extra buoyancy. If you feel more floaty in your salt pool, it is not the salt.

Are there pools filled with actual seawater?

There have been. San Francisco’s Sutro Baths held seven swimming pools, and “During high tide, the Pacific Ocean could fill the 1.7 million gallons of water required for all the pools in one hour,” according to the National Park Service. That is a true saltwater pool, at ocean strength. A natural tide pool on a rocky shore is a different thing altogether.

A backyard salt pool is neither. It is tap water with about 3,200 ppm of added sodium chloride, which is why the kind of salt you add matters. See what solar salt is before you buy any.

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