When and How to Shock Your Swimming Pool

Updated on November 13, 2017
Barack James profile image

Barack is a chemical engineer with a knack for pool chemistry. He has been in the pool maintenance industry for 8 years.


When, Why, and How to Shock a Swimming Pool

Shocking a swimming pool, also known as super chlorination, is a very important process in pool management which every pool owner should understand and know how to do it, when to do it, which chemicals to use, and the best time of the day it should be performed.

Most people don't think about shocking (super-chlorinating) until they have a cloudy swimming pool water or pool algae. But problems can be avoided if you do it regularly.

Before I changed my non-saltwater (chlorine-based) pool to saltwater two years ago, I used to super-chlorinate regularly, but there is no single factor that determines how frequently it should be done.

Basically, a chlorine-based pool needs more maintenance than a saltwater pool. Unless there is algae outbreak or contaminants such as oil and soil, a saltwater pool does not need much shocking.

This is because saltwater pools use chlorine generators to produce a chlorine compound similar to the regular chlorine added into the pool during shocking.

Chlorine generators can be adjusted to increase the amount of chlorine in the pool; however, this is not shocking and may be done only prior and/or after heavy pool usage to maintain chlorine at the recommended levels.

Literally, shocking a pool is adding a specific amount of free chlorine into the swimming pool when free chlorine reads below the recommended level of 3 ppm. This is necessary to kill bacteria, contaminants, and other organisms in the water.

The right question is not after how long you need to shock your pool, but when and why you need to shock your pool. This article includes first-hand information from my experience of more than 5 years with chlorine-based pool, on when, how, and why to shock a swimming pool, the best chemicals to use, and the best time of the day to perform it.

Note: If you are in a hurry and need a quick guide, I understand: Scroll down to the bottom and you will get a quick guide and a detailed video.

1. Shock When Pool Water Temperature Rises

Bacteria and other dangerous organisms such as algae thrive in warm water. On the other hand, chlorine effectivity is affected when the pool water temperature rises above the recommend level.

As such, it is appropriate to keep suitable pool water temperature at recommended levels of 80-84° F. You can use a pool thermometer to measure water temperature; if the temperature reads higher than the required levels, it is time to shock your pool.

2. Shock When the Free Chlorine Level Goes Down

The quantity of free chlorine in the water should be 3 ppm, which should also be very close to total chlorine. Combined chlorine should always be maintained below 0.5 ppm, of 0.0 ppm if possible.

As the free chlorine dissolves in the water, chloramine (combined chlorine) forms, which is ineffective chlorine and cannot kill harmful bacteria and organisms, and cannot sanitize water to remove inorganic matters in the water.

Most test strips available on the market only measure free chlorine, but you also need to know values of total and/or combined chlorine (free and combined chlorine) before shocking your pool.

I use the Lamotte ColorQ Pro 11 digital pool water test kit. I like this equipment since it's accurate, easy to use, and measures other pool chemicals and stabilizers such as pH, Bromine, Cyanuric acid, and Calcium hardness.

Important to note: Improper composition of pool chemicals in the water, especially pH and Cyanuric acid, affects the effectiveness of chlorine, so you need the best equipment for exact readings of chemicals. Don't try to guess using test strips.

If you find that chlorine level is lower than 3 ppm, it is time to shock your pool.

3. Shock After Heavy Rains

Shocking after it rains may not always be necessary, but to be on the safe side, it is advisable to do it after a heavy rain which may carry contaminants and cause big problems if not taken care of in time.

  • First, before shocking, cleaning, or adding any chemicals after heavy rains, ensure that your pool water level is reduced to the normal quantity, as long heavy rains may increase pool water levels.
  • The most affected pool chemical by heavy rains is the pH levels. However, when the rain is acidic and alkalinity levels are within the required range, pH concentration will be protected and all you will have to do is to adjust alkalinity levels.
  • Most important: Total alkalinity (TA) is very destructive when it get out of balance. Closely watch the level of total alkalinity so that it does not exceed the recommended levels of 80 ppm to 120 ppm. Here is how to lower total alkalinity. To raise TA, you can use baking soda.
  • When it rains heavily, particles and leaves may be carried into the pool, so it is good pool hygiene to clean it using large leaf net to remove these particles and leaves.

4. Shock During Extended Periods of Hot Weather

During hot weather, the water temperature always rises above optimal temperatures. This affects the effectiveness of chlorine, and you will find that this is the period when pool bacteria and other organisms such as algae find their way into your pool.

Make it a routine to shock your pool regularly during hot weather to prevent these harmful bacteria and algae from thriving. You can use chlorine stabilizer such as Cyanuric acid to prevent direct UV light from consuming your chlorine at a higher rate.

5. Shock When the Pool Is Heavily or Frequently Used

Chlorine levels reduce drastically when many active swimmers use a pool frequently. You need to measure total amounts of chlorine after heavy swimming, especially in commercial or public pools, and then shock the pool as required.

To find out what amount of free chlorine or any other chemical to add to a pool, I use this pool calculator (by Trouble Free Pool) which allows me to enter the reading I get from from my water test equipment, and bam, I find the correct value of chlorine to add.

For saltwater, controlling the chlorine levels is easier. All you need to do is raise the saltwater chlorine generator (SWCG) to boost free chlorine prior and/or after heavy usage.

What is the Best Time of Day to Shock a Swimming Pool?

Ultraviolet (UV) light from direct sun greatly affects chlorine. Because of this, the best time of the day to shock a swimming pool is in the evening when the sun has gone down.

Which Chlorine Is the Best?

Basically, there are four types of chlorine shock and each has its pros and cons. The type of pool shock to use in your pool will depend on your pool requirements and some other factors.

Remember, using poor quality chlorine shock will always bring more problems like white scum or foam inside the pool. Below is more detailed information on different pool shocks available. You can use this information to decide which is best for you.

1. Calcium Hypochlorite

  • Calcium hypochlorite is the cheapest type available in the market.
  • It has the highest chlorine percentage (65%).
  • Use calcium hypochlorite if you need to add calcium to the pool water. It needs to be pre-dissolved before it is used.
  • Use it at night or in the evening to protect free chlorine from UV light from the sun.
  • After treatment, you need to wait for 8 hours before using the pool.

2. Lithium Hypochlorite

  • Lithium hypochlorite has the lowest percentage of chlorine (35%) and is the most expensive type on the market.
  • To use lithium hypochlorite, you don’t need to pre-dissolve it.
  • Use it at night or in the evening to protect free chlorine from the direct sunlight.
  • You must wait for 8 hours before swimming in the pool.

3. Di-Chlor (Granular Chlorine) Pool Shock

  • Di-Chlor or granular chlorine pool shock has 60% chlorine concentration.
  • It contains Cyanuric acid and should be used carefully.
  • Di-Chlor doesn’t need to be pre-dissolved.
  • Add it at night or in the evening to protect free chlorine from UV light from the sun.
  • Wait for 8 hours before swimming.

4. Pottasium Peroxymonosulfate (Non Chlorine Pool Shock)h

  • Pottasium peroxymonosulfate is a non-chlorine pool shock used mainly in Bromine swimming pools. However, it can also be successfully used in chlorine-based systems.
  • It doesn’t need to be pre-dissolved.
  • You can use a swimming pool shocked with potassium peroxymonosulfate after only 15 minutes, unlike the others that make you wait longer.
  • It can be added at any time as it is chlorine-free and cannot be affected by direct sunlight like the rest.

How to Shock a Swimming Pool

Shocking or super-chlorinating is not difficult and anyone can do it when directives are followed:

  1. Choose (see types listed above) and prepare shock chemicals as required. Some need to be dissolved before use.
  2. Fill a bucket with 5 gallons of water from the pool, which is about or equal to 19 liters.
  3. Add the prepared pool shock into the bucket. Never add water to the pool shock; do it the other way around.
  4. Stir well to mix.
  5. Pour the mixture slowly around your pool.
  6. Refill the bucket with water and stir again to dissolve any residual chemical in the bucket; pour this diluted mixture slowly again around the pool.
  7. Leave the pool for recommended hours before swimming, depending on the type of chemical you use.
  8. Measure the amounts of chlorine to make sure it is 3 ppm or slightly less before swimming. It is dangerous to swim in a pool with high chlorine concentration. You can use chlorine reduction reagents when necessary.

Which Type of Swimming Pool do you Manage

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How to Shock a Swimming Pool

Sources of the Information

This information is based on my experience as a chemical engineer, personal pool maintenance experience of more than 7 years, and standard rules and regulations by relevant bodies.

Questions & Answers


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      • Barack James profile image

        Barack James 7 months ago from Green City in the Sun

        Hi Robert, a swimming pool needs shocking regular all seasons including fall. Before shocking a pool during the fall, you have to do some cleaning using a net and vacuuming equipment to remove dirt. A 20 x 40 pool will not require much chlorine shock. However, the best thing to do is to take the reading of free chlorine level in your water on a regular basis and ensure it is always between 2.5 ppm and 3 ppm, which means your combined chlorine (chloramine) should not exceed 0.5 ppm at any time. Generally, 0.00013 ounce of chlorine per gallon will raise chlorine level by 1 ppm. You can refer the following link for more about how much chemicals you need to add to your pool : https://www.troublefreepool.com/content/114-poolma...

      • profile image

        Robert Brown 7 months ago

        Have you heard of shocking your pool in the fall? If so how much liquid for a 20 x 40 vinyl in ground pool.

      • Barack James profile image

        Barack James 8 months ago from Green City in the Sun

        Welcome Ashly, I wish you good luck...

      • profile image

        Ashly 8 months ago

        Thank you so much for the response! It was very helpful. We bought shock and algae guard. We bought the brand hth super. Wish us luck!

      • Barack James profile image

        Barack James 8 months ago from Green City in the Sun

        Hi Ashly, the green stuff is algae. Do more scrubbing of the walls and then use algaecide or shock the pool with enough chlorine to kill the algae. You can then vacuum the pool or use a net to remove the scrubbed aglae. Here is more on how to get rid of algae: https://dengarden.com/swimming-pools/clearing-up-a...

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        Ashly 8 months ago

        Hi, great article. Our pool is 1 year old and we have had a Huge heat wave this summer. 3 weeks over 100.

        We have green growing on the walls only. I scrub the walls every day now. We don't have green water. I do the chemicals so far. They read perfect. I shocked the pool last week but, the green keeps coming back. Any suggestions for the green walls? Thank you!!

      • Barack James profile image

        Barack James 8 months ago from Green City in the Sun

        Hello FlorencePonk, sorry for the late response. You can use Muriatic acid to lower both pH and Alkalinity in your pool. You need to be careful while adding Muriatic acid so that your Alkalinity is kept between 80 and 120 ppm for your vinyl liner pool. However, in the process, your pH may go down below the recommended level of 7.2: In that case, you will need to aerate your pool to raise the level of pH back to the normal range between 7.2 and 7.8 depending on where you keep your pH. Here is more detailed information on how to balance pH and Alkalinity: https://dengarden.com/swimming-pools/Lowering-Tota...

      • profile image

        FlorencePonk 8 months ago

        Hi, I'm really new to taking care of a swimming pool and this year I only have one of those "Summer Waves" 10ft by 30" vinyl pools with the inflatable ring around the top.

        I've just been using some 3 in 1 poo, and spa test trips, which in most cases have been showing me that everything is okay, however now the Ph level and alkalinity are just very, very slightly high and I can't find any information on how to just tweak those levels just a tiny bit in such a small pool to bring it back down to normal.

      • Barack James profile image

        Barack James 9 months ago from Green City in the Sun

        Hi Amanda, sorry for the trouble. Green pool means you have green algae; I hope you got the link to a complete guide on how to get rid of algae in this article. Wish you good luck!

      • profile image

        Amanda Lopez 9 months ago

        Thanks for all this good information; this is a very small pool but is sad to find it green when my grand kid want to ge in it Im a single mom and never own a pool.

      • Barack James profile image

        Barack James 10 months ago from Green City in the Sun

        Hi Gerri, this article was published purposely for individuals who are green in shocking a swimming pool. The article is also divided into different sections and there is a clear guide on how to shock your pool at the bottom section. It also includes a detailed video at the end showing you how to shock your pool if you need a quick guide.

      • profile image

        Gerri Taylor-Jordan 10 months ago

        This article is much too wordy. I just wanted some quick, down and dirty advice on how to treat a previously clear pool which turned cloudy after it had its weekly cleaning/treatment.