How to shock chlorinate your pool

IMPORTANT:  If you are shocking your pool (sometimes called superchlorination)  to remove high combined chlorine levels then you must raise the free chlorine level significantly.  Putting in a little extra chlorine will not help at all.  It will actually make the problem worse and could make your pool unsafe to swim in. 

Shock Chlorination Formula

The shock chlorination formula is as follows:  Breakpoint Chlorination is when you have enough free chlorine (FC) to shatter the molecular bonds of chloramine.  To do this, you need to add ten times the amount of combined chlorine to hit this point. Yes, that’s 10 times.

For example,  If your pool shop water test shows that your free chlorine is 3ppm and your total chlorine is 5ppm then this means you have 2ppm of chloramines in your pool water.  So, to remove the chloramines you will have to add enough chlorine to raise the free chlorine by 20ppm.  Yes, 20ppm! If you do this then the chloramines will break up and will no longer be in your pool water.

  1. Turn on the pool pump.
  2. Calculate the amount of chlorine you need to add to reach breakpoint chlorination (ex. 16 litres)
  3. Sprinkle or pour the chlorine evenly around the edges of the pool.
  4. Let the shock circulate for at least 6 hours.
  5. Test the water to make sure the chlorine level is good (1-3 ppm) before you use the pool again.
  6. Congratulations your pool water now has no chloramines and is safe for swimming.
Liquid chlorine drum


Turn on your pool pump and have the water tested.  This test 
will tell you the amount of chlorine you need to add to
reach break point chlorination.


Add the liquid or granular chlorine evenly around the pool. 
Then let the water circulate for at least 6 hours and
brush the pool in the areas where the chlorine was added.


Test the water to make sure the chlorine is within the right range and
start enjoying your safe swimming pool water.

Why shock chlorinate your pool?

We shock chlorinate pools for a few reasons:

  • To kill bacteria and algae.
  • To remove chloramines.
  • To clear cloudy pool water.
  • To address severe contamination incidents in the pool, such as after a storm or fecal incident.

Killing bacteria and algae

Chlorine is a very effective disinfectant, but it can be broken down by sunlight, organic matter, and other factors. When chlorine breaks down, it forms chloramines, which are less effective at killing bacteria and algae. Shocking the pool raises the chlorine level high enough to kill even the most resistant bacteria and algae.  If you have black spot algae in your pool click here.

Removing chloramines

Chloramines are responsible for the strong chlorine smell that is often associated with swimming pools. They can also irritate the eyes, skin, and respiratory system. Shocking the pool removes chloramines and makes the pool water more comfortable and inviting.

Clearing cloudy pool water or contaminates

Cloudy pool water can be caused by a variety of factors, including algae, bacteria, and metal ions. Shocking the pool can help clear cloudy water by killing algae and bacteria and oxidizing metal ions. If the pool becomes severely contaminated with bacteria or other pathogens, such as after a storm or fecal incident, shocking the pool may be necessary to make the water safe for swimming again.

How often to shock chlorinate

How often you need to shock your pool will depend on a number of factors, including the size of the pool, the amount of use it gets, and the climate. A good rule of thumb is if the pool is used a lot or if there is a lot of algae growth.  Usually a few times a year is more than enough.

How much chlorine do I need to shock chlorinate my pool?

Let’s say your pool is 40,000 litres.  Your water test shows 3ppm of free chlorine and the total chlorine is 5ppm.  This means that you have 2 ppm of combined chlorine or chloramines (5ppm-3ppm=2ppm).  To remove this 2ppm of chloromines, we need to add 10 times this amount of free chlorine.  So 20ppm is what we need to add to the pool (2ppm X 10).  In this example, you would need 1.4kgs of stabilized chlorine (or 8 litres of liquid chlorine)  to raise the free chlorine by 20ppm.  Your pool shop has these products and will also be able to tell you how much chlorine you need to add to shock the pool properly.  This calculator will help you know how much you need to add based on your water test.

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Michelle JohnstoneMiami - Gold Coast
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Stephen SedgwickKellyville - Sydney
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Stephanie Johanson
Stephanie JohansonToongabbie - Sydney
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Our top 5 shock chlorination tips

1. Don’t shock your pool during the day when it’s sunny. The heat will make the chlorine disappear too fast.

2. If you have a vinyl liner pool, don’t add too much shock at once. It could damage the liner.

3. If you want to swim right after shocking the pool you can use a non-chlorine shock.

4. After shocking your pool, brush the walls and bottom to get rid of any algae or debris.

5. Unless you are using the pool a lot, you won’t need to shock it too often.

Shock Chlorination