What Is Pool Shock?
“Shocking” a pool involves adding a powerful oxidizing agent (usually chlorine) to break down organic compounds and ensure that the pool is properly disinfected and safe for swimming. That’s the short version, we’ll get more technical in a moment. Pool shock treatment is also referred to as super-chlorination or super-oxidization.
Pool chemistry in brief
When chlorine is added to water as part of the normal pool sanitation regime, it reacts with the water to form hydrochlorous acid. It is this chemical that acts as the primary disinfecting agent in the pool. When chlorine combines with ammonia and nitrates (found in normal pool contaminants such debris, sweat, skin cells, leaves or bird droppings), it forms a combined chlorine compound called chloramine or chloramines. The terms “combined chlorine” and “chloramines” are often used interchangeably.
Still not technical enough for you? Then read more on our breakpoint chlorination page. It provides a more in-depth look at pool water chemistry and chlorination.
It is the build up of chloramines that swimmers will most often notice in a pool, complaining of a “chlorine smell” overhanging the pool surface as well as eye and nose irritation. The smell is a sign that you need to shock treat the pool in order to increase the free chlorine levels (i.e. chlorine that is not combined).
Most experts recommend shocking or super-chlorinating the pool to 10ppm on a regular basis (weekly or every two weeks) during the summer months or when the pool is being used. See more on dosage below.
Choosing the best pool shock for your needs
Chlorine vs non chlorine – The main advantage of using a non chlorine shock (these use potassium monopersulphate rather than chlorine as the oxidizing agent) is that you have less pool down-time. Generally you should be able to use the pool 15-30 minutes after treatment but always follow the specific product instructions. Bear in mind however than while non chlorine shock will oxidize ammonia and nitrates, removing the chloramines, it does not increase the level of chlorine sanitizer in the pool. Chlorine free products are popular for spas, hot tubs, bromine sanitized pools and pools with delicate surfaces that are prone to bleaching.
With chlorine based shock it is vital to ensure that the chlorine returns to safe levels before swimming is resumed. And that takes time. Many pool experts recommend shocking in the evening, running the pump and filter overnight and checking chlorine levels in the morning.
A number of different shock treatments are available with the best pool shock products showcased at the top of this page. When it comes to chlorine shock, the key criteria to look out for are available chlorine levels (the higher the better) and whether the product is quick dissolving.
Calculating pool shock dosage – Calculating shock dosage can be complicated and will depend on (1) the pool shock product you are using (2) the size of your pool, (3) the combined chlorine (chloramines) and free chlorine levels and (4) whether you have specific problems with the pool such as cloudy water or a build up of algae. It follows therefore that the guidance below is only a rule of thumb.
Your starting point is to determine the number of gallons of pool water you have in your pool. If your pool is a rectangular or square shape with a flattish bottom then the calculation will be simpler than if you have a custom shaped pool. But here is a handy calculator to get you on your way.
Shocking a pool can be achieved with any form of chlorine – or by using potassium monopersulphate as the oxidizer – as long as it is added in the correct dosage.
Generally you want to shock the pool to raise the chlorine up to around 10ppm. Remember that shocking a pool is an all or nothing process and it will not work if you do not reach breakpoint chlorination.
Dosage recommendations for calcium hypochlorite, lithium hypochlorite, dichlor and non-chlorine shock products. – These assume that you are carrying out a routine shock procedure and have no extreme conditions such as excessive chloramine build up, cloudy water or presence of algae. If your combined chlorine levels are high you may need to add more shock treatment so it is always a good idea to test this first with a good quality test kit. You should aim to introduce 10 times the potential of the level of the combined chlorine in order to reach the breakpoint chlorination point.
- If you are using calcium hypochlorite with 65-68% available chlorine then 1lb per 10,000 gallons is a good guide. Dilute it fully first and read the product instructions carefully. If you have delicate pool surfaces take extra care when using calcium hypochlorite as it may cause bleaching.
- If you are using lithium hypochlorite with 35% available chlorine then the basic guideline is to add around 2lbs per 10,000 gallons for a routine shock treatment. This is because the percentage of available chlorine is lower. Lithium is a good alternative to calcium hypochlorite if you are in a hard water environment where the pool is susceptible to scale. As it dissolves quickly it is also a good options for pools with delicate liners. But it is more expensive.
- If you are using liquid chlorine then calculate around 1 gallon per 10,000 gallons for most brands. But read the product details carefully as the dosage amount will depend on the available chlorine percentage.
- If you are using dichlor with 63% available chlorine then 1 lb for 10-12,000 of water is the general guidance for normal conditions. Dichlor contains cyanuric acid which helps stabilize the chlorine and prevent its breakdown in sunlight. This is another good alternative to calcium hypochlorite but is more expensive. It is also likely to result in less initial clouding of the water after immediately application.
- If you are using non-chlorine shock then you are looking at around 1 lb per 10,000 gallons.
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