Calcium Hypochlorite: All You Need to Know

Calcium Hypochlorite Tablets


As a pool owner, you will have to deal with various kinds of chemicals to keep the water clean and sanitized. One of these chemicals is calcium hypochlorite, and the chances are you have already used it as a pool shock at some point.

Besides shocking, calcium hypochlorite or cal hypo, as many pool owners like to call it, is highly effective at regular pool sanitization. However, if you are going to use it, you have to know what it is all about, from how it works to the best way to use it when cleaning your pool.

What is Calcium Hypochlorite? 

Calcium hypochlorite is an inorganic compound with the chemical formula Ca(ClO)2. It mostly comes in tablet or powder form and bright white color. Also, it has a strong chlorine smell, and it has a more regular chlorine appearance.

With calcium hypochlorite, you get a higher percentage of free chlorine, typically between 65% and 75%, which explains why it is a standard pool shock. Additionally, it is a highly alkaline substance with a pH of 11 to 12.

Besides the higher chlorine content, calcium hypochlorite also includes a lot of calcium. For every 1 ppm (parts per million) of chlorine the compound adds to the water, it will also increase the amount of calcium by 0.8 ppm, and so it always leaves you with increased water hardness.

Calcium hypochlorite will not contain any stabilizer to prevent it from breaking down under the sun, whether you get it in powder or tablet form.

How Does Calcium Hypochlorite Work in Pools? 

When you add calcium hypochlorite to the pool water, it reacts to form hypochlorous acid (HOCl) and other byproducts like calcium and hydroxide. Like with most other forms of chlorine, it is the hypochlorous acid that will clean the pool water.

Hypochlorous acid disrupts the functioning of microbes and organic contaminants in pool water by attacking their cells. It bonds with enzymes and a few other cell components, hence destroying the contaminant’s internal workings, which kills them and leaves the pool water sanitized.

Additionally, it is a strong oxidizer. Therefore, it also oxidizes some of the contaminants and renders them harmless. Since calcium hypochlorite does not contain any stabilizers, it is best to use it at night to ensure it does not break down.

Pros and Cons of Calcium Hypochlorite 

When trying to decide whether to give calcium hypochlorite a try or not, understanding its pros and cons will make your decision an easier one.


1. Higher Levels of Free Chlorine: With up to 75% free chlorine, calcium hypochlorite is quite a strong sanitizing agent. Hence, you will not need to use a lot to get the chlorine in the water to the level you want.

2. Fast Acting: Since you will be adding large doses of pre-dissolved chlorine in the pool when using calcium hypochlorite, it will be fast-acting. You can use it to raise the amount of chlorine in the pool super-fast, which is very useful for pool shocking.

3. Easy to Transport: Calcium hypochlorite will come in powder or tablet form, making it easy to transport. It often comes in 1-pound bags, and the DryTec 1-1901-24 Calcium Hypochlorite Granular is a perfect example of this. Additionally, it is easy to store, and you can keep it for up to 3 years.

4. More Affordable: When you consider the amount of free chlorine in calcium hypochlorite, it is one of the most affordable pool sanitizers. It probably only comes second to sodium hypochlorite when it comes to affordability.


1. Does Not Contain Stabilizer: Calcium hypochlorite does not contain pool stabilizer (cyanuric acid), which means it will break down and dissipate fast if you use it for outdoor pools under direct sunlight. If you do not add a stabilizer, you can lose up to 90% of the free chlorine in your pool in just 2 hours.

2. Increases pH and Alkalinity: The higher pH and presence of a high amount of calcium (up to 35%) in calcium hypochlorite means it is more likely to raise your water pH and alkalinity. Hence, there will be more work for you as you need to bring them back down afterward.

3. Requires Pre-Dissolving: You need to pre-dissolve calcium hypochlorite before adding it to the pool, which is an extra step and hence more work for you.

How to Use Calcium Hypochlorite 

What You Need

  • Calcium hypochlorite (actual amount depends on pool size)
  • 5-gallon bucket
  • Stir stick
  • Safety gear (gloves, goggles, and protective clothing)


Step 1 – Gather Supplies and Wear Safety Gear

The first step seems obvious, but it is still crucial, and you should not overlook it. You need to make sure you have everything you will need for the job and wear protective gear, as calcium hypochlorite poses a significant safety risk if not handled properly.

Step 2 – Test the Pool Chlorine Levels

Next, you have to test the chlorine levels in the pool water. It is best to use a digital tester like the AquaChek NP207 to determine the amount of free chlorine and not just the total chlorine. Test strips will not give you these specific results.

Step 3 – Determine How Much Calcium Hypochlorite You Need

Most calcium hypochlorite manufacturers provide a chart on the packaging to guide you, and so it should be easy to determine how much you will need to add to your pool.

Step 4 – Mix it with Water

When you know how much you need to use, the next step is to dissolve it in water. Here you should fill up the 5-gallon bucket to around three-quarters full and then add the calcium hypochlorite a pound at a time. Keep stirring gently to ensure it dissolves completely.

Step 5 – Pour into the Pool

Walk around the pool and pour the mixture around its perimeter. Once done, you should not use the pool for at least 8 hours to give the calcium hypochlorite enough time to work.

Bottom Line 

Calcium hypochlorite provides one of the best solutions for keeping your pool water free of all kinds of contaminants. Although it works best as a pool shock, you can still use it for regular sanitization, provided you combine it with a good pool stabilizer.

Also, it is essential to make sure you understand its advantages and shortcomings before you decide to give it a try and know how to use it correctly.

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