How Long Does Chlorine Stay in Pool Water?

Is there anything better than jumping into a pool on a hot summer day? Swimming is an excellent activity for everyone, no matter their age because it is fun and can be a great workout. However, standing bodies of water are susceptible to growing bacteria and microorganisms that can be harmful, requiring pool chemicals like chlorine to be added to keep the pool safe.

Chlorine stays in pool water for an average of four and a half days. However, various factors such as UV exposure, hours of use, and the presence of outside substances, can influence how long the chlorine lasts. 

The rest of this article answers common questions about chlorine in pools, so keep reading if you have questions about your pool maintenance and chemistry! You’ll be equipped with the information you need to be a responsible pool owner by the end of this article.

chlorine test kit

Factors That Affect How Long Chlorine Lasts in Pool Water 

The general consensus for how long chlorine will stay in an outdoor swimming pool is four and a half days, after which the chlorine is depleted to an immeasurable level. However, there are several factors that can speed up its depletion.

UV Exposure

The chlorine content of indoor swimming pools may last longer than that of outdoor pools. It is because sunlight can speed up the breakdown of the chemical.

If you live in a sunny area, especially at the peak of summer, it could take as little as two hours for the sun’s powerful UV rays to break down the chlorine and release it into the atmosphere in a gaseous form.

Frequency of Pool Use

How quickly chlorine degrades also depends on how often the pool is used. The more frequently the pool is used, especially by multiple swimmers, the more outside substances enter the pool and interact with the chlorine.

Presence of Contaminants

Chlorine reacts with all substances brought in by swimmers and therefore gets used up quickly if lots of swimmers are bringing in contaminants such as lotion, hair products, sunscreen, oils, and more. The chlorine reacts with these substances to create chloramines, which result in the swimmer’s red eye. 

The presence of leaves, insects, and dirt in the pool can also exhaust the chlorine, so it is vital to keep your pool water clean of these substances. 

Checking Chlorine Levels in Your Swimming Pool

You must check your pool twice or three times a week to ensure that the chlorine level is between 1 and 3 ppm. You’ll need a pool test kit to check your pool’s chemistry. 

I recommend the Poolmaster 2260 5-Way Swimming Pool and Spa Water Test Kit from Unlike other kits, this one comes with five color chips that are clear and easy to read—the kit tests for chlorine, bromine, pH, acid demand, and total alkalinity. 

You can calculate how much chlorine to add to your pool based on its size. Here’s how: 

  1. Measure and multiply the width, average depth, and length of your pool. 
  2. Multiply this value by 5.9 if your pool is round, 6.7 if your pool is oval, and 7.5 if your pool is rectangular. This answer tells you how many gallons of water your pool holds. 
  3. Multiply the number of gallons in your pool by 0.00013. This determines how much chlorine you need in ounces.

For example, if your pool holds 5,000 gallons (18,927 liters) of water, you’ll need 0.65 ounces (18.43 grams) of chlorine to get your pool to a healthy level. 

It is generally safe to swim in your pool 2-4 hours after adding chlorine, but some chlorine products have different requirements. Ensure that you read the instructions to know how soon you can take a dip in your healthy pool water. 

Why Do Pools Need Chlorine? 

Pools need chlorine to prevent the growth of algae and keep the pool water safe for swimming. Chlorine attacks and kills bacteria and microorganisms that could be harmful when ingested or gotten in contact with the eyes or skin.

Chlorine breaks into two molecules: hypochlorous acid and hypochlorite ion, and these molecules attack and kill the lipids in microorganisms. Without chlorine, these microorganisms can cause numerous infections, including gastrointestinal, skin, ear, eye, and wound infections. 

Maintaining pool chemistry with the proper chlorine levels is essential for responsible and safe swimming. The pH of the water in your pool should be between 7.2 and 7.8, as recommended by the CDC. This level of pH prevents the water from irritating the eyes and skin. The pool should also have a chlorine level of 1-3 ppm.   

To ensure that your pool is at the right level, as discussed, you should test it 2-3 times a week and adjust your pool chemistry as needed. 

How much chlorine is used in the pool is called the chlorine demand, and it depends on your pool environment. Chlorine demand increases depending on how many contaminants swimmers bring into the pool, including sunscreen, lotion, hair products, sweat, body oil, and urine. 

How To Help Pool Chlorine Last Longer

There are some ways to reduce how much and how quickly chlorine is depleted while still maintaining healthy pool chemistry. Here are some tips: 

  • Have all swimmers shower before entering the pool. Showering before getting into the pool is an excellent way to wash off contaminants that would use up chlorine. 
  • Use a pool cover. The sun breaks down chlorine, so if you use a solar pool cover that blocks the sun’s UV rays, you’ll reduce how quickly the sun breaks down the chlorine. 
  • Ensure that all children have sufficient bathroom breaks. Urine increases chlorine demand, so if children are swimming in your pool, make sure that they are taking enough bathroom breaks and aren’t urinating in the pool. 
  • Use a chlorine stabilizer. Chlorine stabilizer is made of cyanuric acid, a chemical that reduces the sun’s impact on chlorine. The stabilizer level should be between 30-150 ppm.   
  • Wear a swimming cap. Hair products degrade chlorine, so it is best to avoid adding these substances to your pool as much as possible. Wearing a swimming cap protects the water from whatever is in your hair. 

Following these tips can help reduce your chlorine demand and therefore reduce how often you need to add chlorine to your pool. For more information on chlorine levels in your pool, check out The Beginners Guide to Pool Water Chemistry.

chlorine tablets


The average length of time chlorine stays in pool water is four and a half days, but all responsible pool owners should be checking their pool’s chlorine level two to three times a week to ensure that the water is healthy for swimming. 

Various factors, such as UV exposure and the presence of outside substances, can impact the pool’s chlorine demand. As long as your pool’s chlorine level is between 1 and 3 pm, you’re safe to swim!