Pool Shock 101 (How to Shock a Pool The Smart Way)

A pool shock (also known as shock chlorination) effectively removes cloudiness and unpleasant smells in your pool. But how to do a pool shock correctly, and how often does your pool water have to be overchlorinated?

Liquid chlorine works best for the pool shock. It is important that the chlorine is not stabilized with cyanuric acid and that the pool pump runs after the shock for 24 hours until the water is clear. If the chlorine level is less than 1.5 ppm, you may swim again in your pool.

This article explains how to apply a pool shock in your swimming pool to kill algae and get rid of cloudy pool water. I will walk you through all the essential steps and tell you the best time to perform a pool shock.

What does a pool shock mean?

Pool owners often hear the term pool shock when there is a problem in their swimming pool.

Be it cloudiness from green pool water or unpleasant smells. The pool shock always seems to be the solution to all the problems.

But what is a pool shock, and what does it do in your pool water?

A pool shock is the overdose of chlorine in your pool water. The over-chlorination is intended to disinfect your pool, remove unpleasant smells and remove cloudiness from algae and other organic substances.

The result is crystal clear and hygienically perfect pool water.

Therefore, the pool shock is important for your pool maintenance.

Before you learn how to shock your pool, you should first know that the chlorine level for swimming pools is regulated according to the CDC (Centers for disease control and prevention). Source.

A distinction is made between free, bound, and total chlorine in your swimming pool.

The basics of chlorine:

  • Free chlorine: The chlorine that is available in your pool for disinfection. The level should be 0.4 ppm in an outdoor pool and 0.8 ppm in an indoor pool. The recommended maximum level is between 1.5 to 2 ppm.
  • Combined chlorine: The reaction of chlorine with ammonia and other ammonium compounds. The level of 0.2 ppm should not be exceeded in outdoor and indoor pools.
  • Total chlorine: The sum of free and bound chlorine.

The corresponding levels ​​for the chlorine content in your pool are determined with a pool water tester.

The chlorine is measured photometrically or according to the DPD method – the DPD method can differentiate between free, bound, and total chlorine.

In the following chapter, I go into more detail about the different types of chlorine and explain which chlorine works best for a pool shock.

The different ways to shock your pool

Beginners often assume that any chlorine can be used for a pool shock.

But that’s not true.

Chlorine is available in various physical states (solid, liquid, gaseous), and not all of them are suitable for a pool shock in a swimming pool.

It is important to know: The chlorine must not be stabilized for the pool shock!

In this table, you will find an overview of the different types of chlorine for private swimming pools and how high the proportion of chlorine is.

Chlorine Chlorine Level State Stabilization
Calcium hypochlorite 65% Tablets/Granulate Not stabilized
Sodium hypochlorite 12% Liquid Not stabilized
Di-Chlor 60% Tablets/Granulate Stabilized
Tri-Chlor 90% Tablets/Granulate Stabilized

I focus only on chlorine in the table because this chemical is most used to shock a pool.

The types of chlorine shown are explained in more detail in the following sub-chapters.

You will also learn about other methods to disinfect your pool.

Shock your pool with calcium hypochlorite

Calcium hypochlorite (chlorine granulate) has been available on the market for private pool owners since 1928 and is still the most widely used today – due to its relatively low cost.

With the use of calcium hypochlorite, the total chlorine level in your pool is increased quickly and permanently.

If it is not stabilized, you can use it also for the pool shock.

Calcium hypochlorite is available from pool retailers as fine granules or powder.

But be careful: Do not confuse the non-stabilized calcium hypochlorite with an already stabilized di-chlorine or tri-chlorine.

What you should remember is that Di-Chlor and Tri-Chlor are already stabilized chlorine.

Accordingly, di-chlorine and tri-chlorine are not suitable for a pool shock.

If you want to shock your pool with calcium hypochlorite, please always use non-stabilized chlorine granules – how this works is explained below in the text.

Shock your pool water with liquid chlorine

The liquid chlorine (sodium hypochlorite) is usually not stabilized with cyanuric acid and is ideal for shocking your swimming pool.

If the calcium level in your pool is too high and you do not want to influence it further, then liquid chlorine is preferable to calcium hypochlorite.

In addition to a pool shock, liquid chlorine is also suitable for permanent disinfection in your pool.

In this case, the liquid chlorine must be stabilized with cyanuric acid.

During use, the liquid chlorine is poured directly into the pool basin without dissolving it in a bucket of water beforehand.

However, splashes occur during this application.

Protect your eyes and skin by wearing protective equipment (mask and glasses).

Tip: It is best to wear old clothes when using liquid chlorine, as the splashes can damage them. Adding the liquid chlorine to the flow of the return nozzle in your pool can minimize acid splashes.

Shock your pool with chlorine gas

Chlorine gas is another alternative to disinfect your swimming pool.

However, the risk of accidents with chlorine gas is much higher, and therefore it is not used in private swimming pools.

In addition, increased safety precautions must be taken.

The chlorine gas was still listed in this post to give you an overview of all the possibilities.

A pool shock with chlorine-free pool shockers

Chlorine is not the only product that can shock (disinfect) your pool.

A pool shock can also be performed with active oxygen.

However, the oxidizing power of active oxygen is smaller than that of chlorine.

The active oxygen should not be combined with chlorine as the effects cancel each other, and a poor degree of disinfection in your pool is achieved.

Advantages of active oxygen in your pool:

  • The active oxygen can be added directly to the pool water.
  • You can shock the pool at any time of the day as the sun’s UV rays do not influence the effect.
  • Active oxygen is a chlorine-free alternative to shock your swimming pool or hot tub.

Active oxygen is more expensive than pool chlorine and therefore not recommended for regular use in your swimming pool.

However, the costs are still limited when used in a hot tub.

Other alternatives that can perform a pool shock are Bromine or Biguanid.

However, when using biguanides, the entire pool must first be emptied, cleaned, and the chlorine level reduced to zero.

How much chlorine to use for a pool shock?

The chlorine level in your pool must be increased significantly so that the shock chlorination succeeds.

But how much chlorine do you need in a private pool?

Note the manufacturer’s information on the packaging. Otherwise, you can stick to the following rule of thumb:

  • For every 50 m³ of water, you need 10 liters of liquid chlorine with 150g/l chlorine content.
  • For every 50 m³ of water, you need 20 liters of liquid chlorine with 80g/l chlorine content.

In the first step, you have to calculate your pool volume. This data is then used to determine how much chlorine should be added to your pool.

Without going into the calculation of the chlorine level, here is an example of how much chlorine is needed for the pool shock.

How much liquid chlorine must be used to shock your pool if the pool holds 40 m³ of water?

2 liters of liquid chlorine – with the higher chlorine concentration of 150 ppm – are sufficient for 10 m³ of water. In this case, 8 liters of liquid chlorine are required to shock your swimming pool.

With a lower chlorine content (80 g/l), that would be 16 liters of liquid chlorine.

Do you want to calculate the exact chlorine content?

No problem.

Warning: It gets a little complicated. We assume 10 liters of liquid chlorine and 50 m³ of water.

  1. 10 liters of liquid chlorine correspond to approx 12.5 kg of chlorine.
  2. 50.000 : 12.5 = 4000. This clarifies that a chlorine level of 250 ppm is present in 50 m³ of water.
  3. The chlorine content in % is multiplied by 250, and the result is the net chlorine concentration – with 20% chlorine, the chlorine concentration would be 50 ppm (for 10 liters of liquid chlorine).

It is much easier to determine the chlorine level in your pool with an electronic water tester.

If you add not enough chlorine to your pool water, no effect may be achieved, and you have to do the pool shock again.

You lose time and spend extra money by shocking your pool the wrong way.

Therefore, learn in the next chapter how to do a pool shock.

Instructions: How to shock a pool

Enough with the theory. Now it’s time to learn how to shock your swimming pool.

This pool shock method is suitable against green pool water, cloudy water, and initial pool cleaning.

If you’re using non-stabilized chlorine, it’s best to wait until early evening to do it.

You know: Unstabilized chlorine is sensitive to UV rays.

Equipment required for a pool shock:

  • Safety glasses: The glasses should protect your eyes from splashes.
  • Rubber gloves: Chemical-resistant gloves to protect your hands.
  • Long clothes: It’s best to wear old, long clothes.
  • Water tester: The water test is always necessary when treated with chlorine.
  • Pool shocker: In the example, chlorine is used, but you can choose freely.
  • Water bucket: Alternatively, a bucket with a large volume is required.

Before we start, calculate your pool volume.

With a square and round pool, this is no problem with the help of formulas for the volume (use Google search).

For pools with oval and free shapes, you have no choice but to estimate the water volume in your swimming pool.

Pool shock with liquid chlorine:

A pool shock is useful for cloudy, milky, and bad-smelling pool water.

  1. Put on your protective equipment to protect your eyes and skin.
  2. Read the manufacturer’s information and measure how much chlorine your pool needs. If you have to calculate the levels ​​yourself, follow my recommendations in point 3.
  3. 10 liters of liquid chlorine with 150 g/l chlorine content for every 50 m³ of water. Or 20 liters of liquid chlorine with 80 g/l chlorine content for every 50 m³ of water.
  4. Pour the liquid chlorine into the stream of the return nozzle in your pool. Alternatively, you can fill the chlorine into a bucket and pour it over the edge into the pool basin.
  5. Let the pump run for 24-36 hours until the water becomes clear again (during this time, it is forbidden to enter the pool).
  6. In the meantime, you can scrub your pool with a pool brush.
  7. Measure the chlorine level after the time has elapsed. You can only go back into your pool when the chlorine level is under 1.5 ppm.

Pool shock with chlorine granulate:

A pool shock with chlorine granules is ideal for surface infestation by algae.

  • Wear a respirator and rubber gloves – ideally also protective goggles.
  • Open the chlorine granulate bucket quickly and move away immediately so that the gases can escape.
  • Read the manufacturer’s information on how much chlorine granulate you must add to your pool. Since we’re over-chlorinating the pool water anyway, you can’t go much wrong.
  • First, turn off the pool pump and sand filter system.
  • Wait for a moment of no wind and generously sprinkle the chlorine granules into your pool with a small shovel without aiming at specific areas.
  • Let the chlorine soak in your pool for 24 hours.
  • Then scrub the pool and let the filter system run through for 24 to 36 hours until the water becomes clear again.
  • Test the pool water. If the chlorine level is below 1.5 ppm, you can swim again.

It may be that the manufacturer stipulates that the chlorine granulate in the bucket must be dissolved beforehand.

In this case, dissolve the chlorine and then proceed like you use liquid chlorine.

Tip: Chlorine is always added to the water and never the other way around. This will prevent splatter, which can cause irritation and bleach marks.

When should you shock your pool water?

Is your pool water milky or completely green? Does it smell unpleasant like chlorine?

Then it’s time for a pool shock to clean your pool.

But when exactly should you apply the pool shock, and does time play an important role in how successful the pool cleaning with chlorine is?

You already know that it is best not to use stabilized chlorine to shock your pool. And you have learned that non-stabilized chlorine quickly loses its effect under the influence of the sun’s UV rays.

For this reason, the best time to perform a pool shock is in the late afternoon, when the sun is low, or in the evening.

You can be sure that the chlorine will develop its full effect during this time and that your pool water will be clean again.

Did you perform the pool shock, and the pool water was still milky or cloudy afterward?

One possible cause is that you poured the chlorine into the pool at the wrong hour.

So pay attention to what time you shock your pool.

Tip: If you use active oxygen or bromine, the time of day does not matter. Both pool chemicals are insensitive to the sun’s UV rays.

How often should you shock your swimming pool?

Many beginners ask themselves how often the pool should be shocked with chlorine.

Many advisors recommend an occasional chlorine shock once a week or every two weeks.

The reason for this is – so it is claimed – that algae and chloramines are to be combated by the occasional pool shock, long before they can be recognized by cloudiness and smells in your pool.

I do not believe in this method of occasional pool shock in a pool.

Often it is just a statement from salespeople or ignorant bloggers to increase sales.

It is unnecessary to shock your pool with chlorine in optimally balanced pool water.

That saves your wallet, your health, and the environment – It feels good, huh?

By regularly adding chlorine tablets to your pool, free chlorine is constantly available for disinfection. Algae and chloramines should then not be a problem.

Then why is an occasional pool shock necessary?

Simply follow the recommendations from my blog for pool maintenance, and you will not need any extra pool shock.

Safety instructions when using a pool shock

The pool shock is an essential part of pool cleaning, and as you have learned, it is not difficult once you understand what is important.

But please always keep in mind that these are dangerous chemicals that can harm your health.

Incorrect storage of pool chemicals also has serious consequences.

That is why you will find safety instructions for using and storing chemicals here.

  • Always wear protective equipment: Yes, it can be annoying to wear protective equipment every time, and many pool owners don’t, but chlorine irritates the mucous membranes and skin when touched. Make it a habit to always wear protective goggles and rubber gloves during a pool shock.
  • Avoid breathing gases: Chlorine forms gas at high temperatures, which you should not inhale under any circumstances. A lot of gas escapes, especially when opening chlorine granulate. Therefore, open the container quickly and move away until the gases in the air have dissipated.
  • Pay attention to the manufacturer’s information: I try – as difficult as it may be – to give you the best tips about the use of pool chemicals, but the manufacturer’s information is always preferable and must be observed.
  • Store pool chemistry properly: The pool chemicals must be stored in a dry and well-ventilated place. Avoid filling into other containers, and under no circumstances should you mix different chemicals outside of your pool – the risk of fire and explosion is present.
  • Open only one container at a time: Empty one chemical container first, then the next if more chlorine is needed.

Questions and answers about the pool shock

You can find more information about the pool shock here.

Are pool shockers dangerous?

As long as the pool chemicals are stored correctly, not mixed, and dosed in your pool according to the manufacturer’s instructions, there is no danger to be assumed.

However, certain chemicals such as chlorine or bromine form by-products in your pool water that can be harmful to your health if they are not regularly checked and broken down.

That is why many pool owners are interested in running the pool without chlorine – you can find more about this topic in my blog article under the link.

Can I go straight back into the water after chlorination?

After a pool shock, swimming is not allowed.

The chlorine level in your water is too high, which can be harmful to people (burning eyes, irritated mucous membranes, etc.).

As a rule, swimming in your pool is possible again after 24 – 36 hours.

It is best to measure your pool water and only reopen your pool when the chlorine level is below 1.5 ppm.

How long does a pool shock last?

Therefore, the pool shock (with chlorine) lasts between 24 and 36 hours.

During this time, the pool water should clear again, and the chlorine level should drop below 1.5 ppm.

Why is my pool water still green and cloudy after a pool shock?

If you have applied a pool shock and your pool is still green and cloudy, this may be because you don’t use enough chlorine, and your pool was not flocculated for a long time.

Another cause would be that your filter system runs under 6 hours/day.

The solution: Try to turn your pump on permanently until the water in your pool becomes clear again.

However, it can also be because the filter medium in your system has not been changed over one year, and the filter performance is bad. In this case, you have to change the sand in the filter system.

If the water in your pool remains cloudy, the multi-valve must be changed, as the water will probably flow back into the pool unfiltered.

What are the options for a chlorine-free pool shock?

The pool shock does not necessarily have to be carried out with chlorine, although it must be said that chlorine has the strongest effect.

Alternatives would be bromine, active oxygen, or biguanides. Active oxygen is very popular as an alternative to chlorine and is gentle on bathers.

Do you have any further questions about how to do a pool shock?

Use the search function or contact me at @contactswimfool on Twitter. I am happy to help you.

Good luck with performing the pool shock in your pool.

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Hi. I'm Max Berg. I've been in the pool industry since 2015 and have always felt drawn to water. I'm the author behind swimfool.com, where I share my years of experience in pool maintenance and give helpful tips on keeping a swimming pool or hot tub clean. My tips reduce the costs of water treatment and protect the environment.

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