Stabilized vs Unstabilized Chlorine: Which One Should You Use?


by Rick Patterson

Unless you’re unfamiliar with basic pool knowledge, you know that they require a sanitizer, usually in the form of chlorine, to keep the water clean.

But chlorine comes in two distinct categories: stabilized chlorine and unstabilized chlorine – both of which can be used in your pool.

So what’s the difference?

What Is Stabilized Chlorine?

The 2 types of stabilized chlorine are sodium dichlor and trichlor.

Sodium dichlor is known for its near-neutral pH level of 6.5. This makes pH level fluctuations in the water less likely. It’s sometimes used as a shock treatment, and is very effective at killing black algae

Additionally, dichlor is also resistant to high water temperatures, which makes it a good type of chlorine to use in hot tubs.

For pools, trichlor is the preferred stabilized chlorine. It has a low pH level of 3, and is 90% active chlorine. Trichlor comes in tablet form which is slow-released to keep the pool water steadily sanitized over a long period. 

How Does It Work?

The main difference in stable vs. unstable chlorine is how they operate.

Stabilized chlorine contains cyanuric acid. Cyanuric acid (CYA) protects chlorine from the sun’s UV rays (think of it like chlorine sunblock).

If an outdoor pool is using chlorine and no cyanuric acid present is in the water, the sun will quickly break it down. This means you’ll constantly be feeding your pool chlorine to keep the pool water chemistry balanced.

So, cyanuric acid prolongs the life of chlorine – about 3 to 5 times longer than unstable chlorine. This is great for you because you won’t need to keep spending money on replacing sun-depleted chlorine!

If you have an indoor pool, stabilized chlorine technically doesn’t need to be used because there’s no sunlight hitting the water. However, it’s recommended to still use small amounts of CYA (20 to 50 ppm). This is because CYA also helps moderate the strength of chlorine, which makes it less harsh on your skin and hair, and you won’t need to replace your swimsuit every year.

When Should You Use It?

Stabilized chlorine is for use with outdoor pools. 

In general, you want to keep the chlorine level at 3ppm (parts per million) at all times for constant sanitization.

We advise you to regularly test your chemical levels to ensure that the water is clean and performing optimally.

Additionally, it’s important to keep an eye on your water’s CYA level when using stabilized chlorine. If cyanuric acid levels get too high, you’ll have a whole new issue on your hands, as the only way to remove it from your swimming pool is to drain/dilute the water.

The Pros & Cons

Stabilized chlorine comes with plenty of upsides, and few downsides.


  • 2-in-1 product
  • Chlorine protected from the sun with CYA
  • Chlorine lasts up to 8 times longer
  • Easier to maintain proper water chemistry
  • Saves money on fewer chemicals


  • Too much CYA can reduce chlorine effectiveness
  • High CYA levels can lead to cloudy pool water
  • Pool must be diluted/drained to reduce CYA levels

What Is Unstabilized Chlorine?

Now that you are familiar with stabilized chlorine, how does unstabilized chlorine differ?

There are a few key differences, but none of them have to do with the chlorine’s ability to keep the pool water clean.

Unstabilized chlorine comes in 3 types: sodium hypochlorite, lithium hypochlorite and calcium hypochlorite.

Sodium hypochlorite is a liquid chlorine normally found in household bleach. It has a high pH level of 13, between 10% and 12% available chlorine, and is already diluted to a safe level for your pool. It’s easy to pour directly into your pool’s skimmer, and it won’t raise the pool’s overall pH level, or its calcium levels. It’s ease of use also makes it the chlorine choice for water parks and large commercial pools.

Lithium hypochlorite is a granular chlorine, allowing for easy dissolution in water. It has much higher available chlorine at 35%, which makes it a good option to use when you need to shock or super chlorinate your pool.

Calcium hypochlorite (cal hypo) is another granular option, but is more widely available in a puck or tablet. This is the most commonly used unstabilized chlorine, as it has anywhere between 40% and 78% of available chlorine in it.

It’s also the most popular chlorine for pool shock, though you should always be aware of your pool’s calcium levels so you don’t overdo it. Additionally, the higher available chlorine percentage in calcium hypochlorite also means it costs more.

How Does It Work?

Unstabilized chlorine is just that – pure chlorine. Unlike stabilized chlorine, it does not contain any cyanuric acid. This means that it has zero protection from the sun’s UV rays, and you will be using a lot of it (and frequently) to keep your outdoor pool disinfected.

Adding unstabilized chlorine is usually simple, but it does depend on if you’re adding liquid chlorine, granules, or pucks/tablets.

Granules are mixed with water and the solution is poured around the perimeter of the pool. Liquid is added in the same way. This allows the return jets a few hours to circulate the chlorine.

Pucks/tablets are added to the skimmer basket, or via a floater for a slow-release effect.

When Should You Use It?

Unstabilized chlorine can be used at any time, but it’s situationally dependent on the pool’s CYA levels. 

If you have a pool that already has CYA in the water, unstabilized chlorine would be the choice to go with.

If you have no levels of CYA in the water, and you want to use unstabilized chlorine, you’ll have to add CYA in separately to provide some protection to the chlorine.

Unstabilized chlorine is also preferred when shocking a pool but it has to be done overnight, as the lack of CYA means the shock would be burned off by the sun pretty quickly. Shock needs at least a few hours to kill everything in the water, and once the sun comes out, it will burn off 1 ppm per hour of exposure.

For outdoor saltwater pools that generate their own chlorine, they don’t make CYA. 50 ppm of CYA will have to be added to the pool water to keep it protected.

If you have an indoor pool, you can use unstabilized chlorine and no CYA, although it’s recommended to keep a low CYA level in the pool to counteract the harshness of the chlorine. Also, as with outdoor pools, if high CYA is already present in the water, unstabilized chlorine is the choice to go with.

The Pros & Cons

Unstabilized chlorine has its benefits but also its drawbacks.


  • Use to shock the pool and kill off bacteria
  • Can be used in indoor pools
  • Comes in liquid/granules/tablets


  • Not protected against sun’s UV rays
  • Have to spend additional money on CYA to protect it

What About Chlorine Tablets?

Adding chlorine to your pool water can easily be done with chlorine tablets.

Tablets are slow dissolving, so you can add them to the pool and not have to worry about the chlorine level for a few days. They steadily chlorinate the pool, giving you the maximum amount of chlorine for your dollar.

They’re designed to be used in floaters, feeders, or skimmers. Never add chlorine tablets directly to the pool as they can damage the vinyl liner or finish.

You’ll also notice some chlorine tablets are multi-functional, with built-in pool clarifier and algae control.

Stabilized Chlorine Tablets

Stabilized chlorine tablets contain cyanuric acid, which will protect the chlorine from being broken down by the sun’s UV rays.

This is especially important in an outdoor pool. With no CYA you’ll constantly be adding lots of chlorine. The most common stabilized chlorine tablet you’ll come across is trichlor.

Unstabilized Chlorine Tablets

Unstabilized chlorine tablets burn up quickly in the sun. Calcium hypochlorite is the most common unstabilized tablet on the market.

You can still use them in an outdoor pool, but you’ll have to add cyanuric acid to the water to protect the unstabilized chlorine. It adds an unnecessary and costly step to your pool maintenance that can be eliminated by just going with a stabilized chlorine tablet.

If you have an indoor pool, with no sunlight hitting the water, you can use unstabilized chlorine tablets without the need for cyanuric acid.

Stabilized vs Unstabilized Chlorine

As you can see, the debate between stabilized vs unstabilized chlorine is wholly dependent on your circumstances.

For outdoor pools, stabilized tablets are best for day to day disinfecting, coupled with weekly shocking using unstabilized chlorine.

However, if you have an indoor pool, you can use unstabilized chlorine tablets on their own just as effectively.

Categories: Pool Care, Pool Chemistry