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Calcium Hypochlorite: The Complete Guide for your pool

Calcium Hypochlorite

Calcium Hypochlorite, also known as cal hypo is one of the most popular disinfecting agents used in swimming pools. Calcium Hypochlorite has specific characteristics and is only one type of chlorine product, it is certainly not the only chemical used in domestic or public swimming pools. Cal hypo is a potent form of chlorine that usually comes in a solid block or tablet providing the user with a simple, straightforward way of adding chlorine to swimming pool water. It is often used to sanitize the pool but it can also be used to prevent algae formation.  

Calcium Hypochlorite Facts 

Calcium Hypochlorite is often preferred by pool owners because it gives the water a sparkling, clear appearance. It also eliminates bacteria effectively and it can also be used in spa water. 

  •   Cyanuric acid is not found in Calcium Hypochlorite, cyanuric acid is a stabilizer that is sometimes found in pool sanitizers
  • Calcium Hypochlorite usually comes in tablet or block form, the tablet version dissolves slowly in water however, the granulated formula is a fast-acting version of Calcium Hypochlorite
  • When you add cal hypo to pool water it sometimes increases the pH and alkalinity of the water
  • You need to use cal hypo with caution as it is a potent oxidizer.

Types of Calcium hypochlorite 

There are various types of cal hypo, they come in different strengths and formulations. For example, 80 percent strength cal hypo is a popular choice but you can also find the 65 percent formula which is the most common choice for pool owners. The 65  percent formula has been the most prominent swimming pool sanitizer since its inception in 1928. 

Calcium hypochlorite dissolves in swimming pool water and converts to hypochlorous acid which is a biochemical that acts as an antibacterial agent. The levels of hypochlorous acid and hypochlorite are related to the overall pH level of the water. The appropriate pH for pool water is approximately 7.2 to 7.8 on the pH scale. The antibacterial capabilities of the cal hypo is largely connected to the presence of hypochlorous acid, the potent acid eliminates harmful germs and algae. 

Aside from acting as a powerful sanitizing agent cal hypo can also be used to treat other pool water issues such as algae formation and it can facilitate the elimination of chloramines in the water. When using calcium hypochlorite, it is necessary to maintain the correct balance of free chlorine, this will help to ensure that the water is properly sanitized. 

One of the primary goals of pool owners when using calcium hypochlorite is to achieve clear, sanitized water. The free chlorine that is produced by cal hypo cleans and sanitizes the water resulting in algae-free, sparkling water. One of the number one causes of bad odors in swimming pools is the presence of chloramines, free chlorine works to get rid of chloramines as well. 

One of the main components in calcium hypochlorite is calcium, there needs to be a certain level of calcium in the water to maintain alkalinity levels and to maintain the correct levels of water hardness. If the balance is incorrect, it results in cloudy water and unwanted scaly residue. 

There are various solutions that can be utilized to reduce scale formation and murky water. Often, draining the pool water and refilling with clean water resolves the problem. Performing a total pool drain is often the best solution because doing this will remove unwanted particles and bacteria from the pool. 

How to Use Calcium Hypochlorite 

Before you start using calcium hypochlorite to treat your pool water, you need to identify the volume of water that you are about to treat. The typical balance you want to achieve when sanitizing your pool is a chlorine concentration of 1 to 4ppm for standard swimming pools. 

Thoroughly read the guidelines on the packaging to ascertain exactly how much product should be used. It also depends on what you wish to use the calcium hypochlorite for, if you want to perform a shock treatment, oxidation or super chlorination. 

There are certain boundaries for testing when it comes to using calcium hypochlorite, the main marker for testing is the level of free available chlorine. The testing method will depend largely on the kind of pool you are testing for example, personal or commercial pool. Prior to administering any pool water treatment, it is necessary to test the pH and alkalinity of the pool water. Also, calcium hardness and cyanuric acid levels should be tested. 


When handling calcium hypochlorite, you should handle it with care and caution. It has potent oxidizing capabilities, it can oxidize various materials such as metals therefore, store and handle with care. 

When it comes to storage, it is vital that you store it in a place with adequate ventilation. The area should also be dry and relatively cool. Store it away from chemicals that might react badly when they come into contact with calcium hypochlorite. 

It is also necessary to refrain from mixing it with other swimming pool chemicals when it is in solid form. For example, cal hypo tablets should not be mixed with chlorinating products or any other chemicals. Never use an old cal hypo bottle to store other chemicals, dispose of the package immediately after use. 

Mixing other agents with calcium hypochlorite could cause adverse chemical reactions, it could even result in fire or the omission of dangerous gasses. 

Pros and Cons of Using Hypochlorite 

Just like any other sanitizing agent or pool water chemical, there are advantages and disadvantages of using them. Calcium hypochlorite is no different, here are some of the benefits and drawbacks of using calcium hypochlorite in pool water. 


It is Powerful 

Calcium hypochlorite is a potent product therefore, you do not have to use large amounts to get the desired result. 

Fast Acting 

Cal hypo works quickly and dissolves fast to get the job done in good time.

No Damage 

If used correctly, it should not cause any damage to your pool tiles. 

Lasts Long 

It lasts for about 2 to 3 years without going bad. 


Calcium hypochlorite is cost-effective, is probably the most cost-effective product for chlorinating pool water, aside from sodium hypochlorite. 


Here are some of the disadvantages associated with using calcium hypochlorite: 

Needs to Be Pre-dissolved 

With calcium hypochlorite, it needs to be pre-dissolved before you can use it. This prevents any type of bleaching from taking place. This is something that is not necessary with alternative chlorine products. 

Can Cause Short-Lived Cloudiness 

If you use large quantities of the powdered cal hypo, it can cause short-lived murky water. This doesn’t last for long but it could be a problem if you need to use the pool straight away.

Tablets Need Feeders 

 Calcium hypochlorite tablets need specially designed feeders to slow down the rate in which they dissolve in the pool water. 

Calcium Hardness 

Using calcium hypochlorite pushes the calcium hardness levels way above the required level. 

Daily Use 

You need to use it daily to chlorinate the pool water, making it an ongoing chore. 

Using Calcium Hypochlorite For Swimming Pool Shocking 

Swimming pool shocking is necessary when there are too many chloramines in the pool water. When this happens it slows down the free available chlorine reducing its overall effectiveness. This buildup of excessive chloramines can cause skin and eye problems for swimmers, therefore, it is necessary to get rid of or reduce the number of chloramines in the water. 

In order to eliminate the excess chloramines from the pool water, you will need to shock the pool water by putting large amounts of chlorine into the water. The recommended amount is 10 times the usual dose, doing this destroys the chloramine molecules, they then begin to oxidize. 

It is recommended that you carry out this process weekly to inhibit chloramine escalation. You can also use this method to eliminate algae build-up or to remove other bacteria or build up from the pool water. Calcium hypochlorite is often used because it is extremely potent therefore, you don’t have to use as much to achieve the required dosage to shock the pool, it also works very quickly, making it an ideal chemical for shocking. One other reason why it is suitable for shocking is that it is not stabilized therefore, it will evaporate quicker, which is necessary when it comes to shocking the swimming pool water

To conclude, calcium hypochlorite is a powerful agent used to sanitize, shock and chlorinate swimming pool water. There are other options out there but some choose cal hypo because it is cheap and strong, which means less is needed and it acts fast. The drawbacks are that special equipment is needed to feed the tablets into the pool water to prevent them from dissolving too quickly. Also, the extra step of pre-dissolving the tablets prior to putting them into the pool is time-consuming. However, it is a multi-purpose product that can be used for a number of things such as shocking the pool, eliminating algae and removing harmful bacteria. 

Posted in pool maintenance

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