Swimming Pool Chlorine 101: What It Is, How It Works, and How to Use It in Your Pool

Chlorine is the most important chemical in most swimming pools. Pool owners know that it keeps their pool in safe working order, but many don’t quite understand how. Knowing the basics of swimming pool chlorine can help you feel more confident in your pool maintenance routine and the products you choose. This guide breaks down what chlorine is, how it works, and the ways you can use it in your pool.

What Is Swimming Pool Chlorine?

When people talk about chlorine in a swimming pool, they're not talking about the chemical element from the periodic table. Pool chlorine is actually a common blanket term for several different chemical compounds. Altogether, these compounds are the most popular pool sanitizers. Unless you own a natural swimming pool, your pool likely contains chlorine.

Pool chlorine comes in three forms.

• Tablets are the easiest to use since they dissolve on their own consistently over time.

• Powder chlorines provide more hands-on control over your pool’s chlorine levels.

• Liquid chlorines tend to cost the least, but require much more effort to achieve the right chemical balance. They are more commonly used in large commercial pools.

What Does Chlorine Do in a Pool?

Pool chlorine is manufactured to keep pool water safe and clean for swimming. When pool chlorine is added to water, it reacts to create the chemical compound hypochlorous acid (HClO). HClO combines with algae, bacteria, viruses, and other dangerous molecules to disable them. Rest easy, though: the doses of chlorine needed to clean a pool won’t harm humans.

This chemical reaction also requires your pool to be at the right pH level. You can monitor and adjust your pool’s pH level with other additives.

Benefits of Using Chlorine in Your Pool 

Chlorine is by far the most popular choice for sanitizing pool water and for a good reason. It works quickly and effectively to kill contaminants in the water. It’s also cost-effective, with a 2-gallon jug ranging around $7 to $10. Since you need about 1 to 3 ounces of chlorine per 500 gallons of pool or hot tub water weekly, one jug will last a long time.

Also, chlorine works regardless of the weather, whereas the chlorine alternative, bromine, doesn’t work as well in sunlight.

Types of Chlorine Pool Chemicals

Sunlight speeds up the chemical reaction of chlorine, causing it to burn up more quickly. Because of this, there are two broad types of chlorine that differ in their cyanuric acid (CYA) content, a substance that slows that burn rate. Both types keep pools clean, but you need to choose the right pool-cleaning chemicals for your needs. 

Stabilized Chlorine

Stabilized chlorines contain CYA, which decreases chlorine’s burn rate in direct sunlight. They are typically only used in outdoor pools that need UV protection. Because the majority of residential pools are outdoors, stabilized chlorines are the most common chlorine type. They’re relatively affordable and easy to use as long as you monitor your pool's CYA level.

Unstabilized Chlorine

Unstabilized chlorine has no CYA and lacks UV protection. It can be used for indoor pools or pools that are shaded. You can also use it outdoors if you add a CYA separately, and it can be used in large doses to shock your pool or quickly restore low chlorine levels in a pinch.

Calcium hypochlorite is the most popular unstabilized chlorine, although vinyl and fiberglass pool owners often choose lithium hypochlorite because it prevents bleaching on those surfaces. Sodium hypochlorite is most common in large commercial pools.

If you're unsure which type of chlorine you should use, or if you have any questions about how to maintain your pool properly, it's best to contact a pool repair pro near you.

How to Add Chlorine to Your Pool

There are several ways to add chlorine to a pool.

• Floating chlorine dispensers are small containers for chlorine tablets that float on a pool’s surface, slowly releasing chlorine. Dispensers are inexpensive and convenient but don't circulate chlorine well. They can also get stuck behind ladders and bleach pool liners.

• Pool skimmer baskets are part of a pool's filter system, usually under plastic covers at a pool's edge. Adding chlorine tablets to a skimmer lets them slowly circulate through the system as they dissolve. Only leave tablets in a skimmer when your pump is on.

• Automatic chlorinators attach to your filter system and release a measured amount of chlorine to your pool at a constant rate you set with a dial. They are the most expensive method of adding chlorine, but also the most effective.

Pool Shocking

Shocking is necessary when chlorine combines with harmful molecules to form chloramines. These compounds cause an overpowering smell, sanitation problems, and skin and eye irritation. Shocking involves adding huge quantities of chlorine to overpower the chloramines and rip them apart.

Salt Water Chlorinators

Even saltwater pools contain chlorine. Saltwater systems have a salt water chlorinator that converts pool-grade salt into chlorine. When comparing saltwater and chlorine pools, saltwater pools do require lower chlorine levels and fewer chemicals.

How Much Does Pool Chlorine Cost? 

An ongoing chlorine shortage has caused prices to surge, but pool chlorine tablets are the most cost-effective option for residential pool owners. You can buy tablets in 25-pound bags for $60 to $70 each. Alternatively, liquid chlorine costs $7 to $10 per 2-gallon jug. 

Alternatives to Chlorine for Disinfecting a Pool 

The alternative to chlorine is bromine. One major downside to bromine is that it loses effectiveness in sunlight, making it an inefficient option for outdoor pools. Plus, it costs twice as much as chlorine. 

Whether you use chlorine or bromine to sanitize your pool, you can go one step further by adding equipment to help keep your water clean. Ozonators, ionizers, UV sanitizers, or mineral water systems kill microorganisms or viruses. If you add one of these, remember it’s only effective if you pair it with sanitizer. 

Nick P. Cellucci contributed to this piece.