Calcium Hypochlorite: What Is It and How it is Used

Written by Michael Dean
January 5, 2024

bucket of calcium hypochlorite

Is it time to shock your swimming pool? While there are a few different methods of shocking a pool, the most popular and cost-efficient way to do so is to use a type of chlorine called calcium hypochlorite (cal hypo). Whether you are a new pool owner or looking to refresh your memory after the winter, I have got you covered!

In this article, I will cover everything you need to know about cal hypo, including what it is, how it works, when to use it, and how to use it.

Main Takeaways

  • Cal hypo is a pool shock and water sanitizer that generally contains 65% to 75% chlorine.
  • You should shock your pool every week, especially after a storm or a pool party.
  • In order to add cal hypo to your pool, you’ll need to test your water, calculate how much to add, mix it in a bucket, and evenly distribute it throughout the pool.
  • Sodium hypochlorite, also known as liquid bleach, may be a better option for you if you live in an area with high calcium content in the water.

What Is Calcium Hypochlorite?

Calcium hypochlorite, or cal hypo for short, is an ingredient in a pool shock container, but it is also an effective water sanitizer itself. In fact, it is often used to clean drinking water, too. This chemical compound has been utilized since the 19th century and is one of the cheapest ways to shock your swimming pool.

This chemical is created when calcium oxide (lime) mixes with chlorine gas. The result is a strong base, usually sold as a granulated solid in stores. It reacts well with a variety of chemical compounds, and it carries a higher pH level. Some of the many reasons it is a popular product are its availability, price, ease of use, and ability to store for long periods.

How Does Cal Hypo Work in Your Pool?

So, how does calcium hypochlorite work? Cal hypo is a sanitizer, meaning it is one of the many chlorine types you can use to disinfect your pool. Most commercially sold cal hypo products contain anywhere between 65% and 75% chlorine in the mixture, which is more than enough to kill algae and bacteria in the water. For every 1 ppm of free chlorine (FC) in your pool, calcium hypochlorite adds about 0.8 ppm of calcium into the swimming pool water.

Cal hypo starts working as soon as you pour it into your pool; the chlorine compounds will encounter the microbes and begin to disturb their functions. It does this by attaching to the enzymes of the microbes’ cells, disrupting all their inner workings and ultimately destroying the cell walls.

For more on the science of your pool, be sure to read by swimming pool chemistry 101 guide.

In The Swim Calcium Hypochlorite Chlorine Granular Pool Shock

In The Swim has a reliable cal hypo shock that is effective and easy to use.

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I may earn a commission if you make a purchase, at zero additional cost to you. This in no way impacts my research process or opinions.

When to Use Calcium Hypochlorite in Your Pool

Calcium hypochlorite usually works most effectively at night. It contains unstabilized chlorine, which quickly dissipates in sunlight. Your best bet is to shock your pool with cal hypo at dusk.

While calcium hypochlorite is an excellent solution to a dirty pool, you should still try to keep your pool clean. Don’t wait for the disgusting chloramines to build up in the pool. Instead, keep a consistent maintenance schedule and regularly add chlorine. I recommend shocking your swimming pool once every week or two, depending on how often you use it.

Circumstances When You Should Shock Your Pool with Cal Hypo

You should obviously shock your pool pretty consistently, but a few unique events will require you to shock your pool.

Storms. If you recently experienced a bad storm in your area, debris may have been blown into the pool, and the excess water may dilute your chlorine levels. A good ‘ol shock should bring everything back to normal.

Pool parties. Another time to shock your pool with cal hypo is after excessive use, such as a pool party. The more people enjoy your pool, the more oils, sunscreens, bodily fluids, and other germs enter your water. Shocking is a great way to ensure you eradicate all contaminants.

Chemical imbalances. You should be testing the pH, alkalinity, and chlorine levels of your pool frequently. If your free chlorine levels drop unexpectedly, you should shock your pool to fix the levels and kill any algae or bacteria.

Why Calcium Hypochlorite is an Effective Solution for Your Pool

Safety should always be a priority for you and your family; it all starts with how clean your pool is. You should shock your swimming pool on a consistent schedule to keep your water clean and safe for the people who use it. Not only will shocking the pool raise the sanitizer level, but it will also re-energize the existing sanitizers, making them more effective.

When chlorine attaches to organic matter like body oils, leaves, debris, and dead skin, harmful chemicals are created. And chlorine cannot perform its regular job effectively when trapped inside this bond. In fact, it may surprise you, but the typical pool chlorine smell you may be familiar with does not mean things are fine; rather, it indicates that a higher level of chloramines is floating throughout the water.

Chloramines are more than an annoying compound that makes your pool smell. They can also cause respiratory problems in humans, which is another reason why it is essential to consistently deploy calcium hypochlorite.

Step-by-Step-Process: How to Shock Your Pool with Calcium Hypochlorite

Ready to shock your pool? Here is my walkthrough of using cal hypo to sanitize your pool water. Besides adhering to the usage and storing instructions on the packaging, the overall process of shocking your pool with cal hypo is simple and straightforward.

Supplies Checklist

  • Calcium hypochlorite
  • 5 gallon bucket
  • Safety goggles
  • Respirator mask
  • Closed toe shoes
  • Gloves
  • Wooden stir stick
  • Testing strips or a liquid test kit

Step One: Gather Your Supplies

The first step is gathering the necessary supplies to perform the shock. Safety gear is always of the utmost importance when dealing with pool chemicals, as they are potent and can harm you if you don’t take the proper precautions.

Step Two: Test Your Pool Water

Next, you should test your swimming pool water. You are testing for free chlorine specifically, which should be around 3 parts per million (ppm).

Step Three: Determine How Much Cal Hypo To Add

After testing your chlorine levels, it is time to calculate the amount of cal hypo shock you’ll need to use. Your bag of calcium hypochlorite likely comes with instructions for how to calculate this. You’ll need to work out your pool volume, the concentration of the cal hypo (generally around 67%), and your current free chlorine levels. Once you have this, you can calculate how much shock to use. Luckily, you don’t have to sweat the math. You can just use my calculator to figure out pool chlorine levels.

Step Four: Mix the Chlorine

After you have measured the ideal amount of solution, you can fill up your bucket to 75% capacity with warm water. Make sure to add the cal hypo to the water one pound at a time and then stir at each round of dumping. Continue to stir this consistently until the sanitizer is fully dissolved in the bucket. Always add chlorine to water and NEVER the other way around; doing so can cause a dangerous chemical reaction.

Step Five: Add the Mixture to Your Water

It is now time to walk around the pool and pour the mixture into the surrounding edges. You should try your best to disperse it evenly and slowly. If you have any leftover residue in the bucket, you can dunk it into the water, mix it gently, and then continue walking and pouring.

Step Six: Be Patient and Let Cal Hypo Work Its Magic

I advise you not to use the pool for at least eight hours. If you go in too early, you can put yourself at risk for itchy skin and eyes or damage to your swimsuit.

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Calcium Hypochlorite vs. Sodium Hypochlorite

Calcium hypochlorite and sodium hypochlorite (liquid bleach) are both proven disinfectants, but there are a couple of differences. For one, cal hypo almost always comes in powder form, while sodium hypochlorite is generally a liquid. Additionally, calcium hypochlorite contains calcium, and sodium hypochlorite does not. Areas with higher calcium hardness levels might not react as well to calcium hypochlorite.

Not only is sodium hypochlorite calcium-free, but it also is much less concentrated, meaning it does not have much available chlorine. Overall, it is a much cheaper product than calcium hypochlorite. That said, if your tap water has higher calcium hardness levels, this might be a better option for you in the long run.

Cal Hypo vs. Trichlor/Dichlor

Cal hypo is also very different from dichlor and trichlor. These are two types of stabilized chlorine, so they are not generally used for shocking but instead for keeping your chlorine levels constant. You can read my comparisons of cal hypo vs. dichlor and cal hypo vs. trichlor for a complete breakdown of the use cases for each.

Health and Safety Considerations for Calcium Hypochlorite

Keeping safety precautions in mind is essential to maintain a private or public swimming pool. You want to keep your pool safe and healthy, but you must always protect yourself first. Chlorine can be a hazardous chemical, so it is important to stay safe when using it. Here are a few health considerations to keep in mind when using cal hypo.

Change Your Clothes When You Come into Contact with Cal Hypo

If your clothes have been contaminated with calcium hypochlorite, you should change into clean clothes. I recommend taking your contaminated clothes straight to the laundry room and not mixing them with other articles of clothing.

Be Prepared with an Eyewash Station

It is also a good idea to have an eyewash station or something similar close by. This way, you can quickly flush out any cal hypo that splashes in your eye when shocking the pool.

Clean Up When Done Shocking

After you use the cal hypo to shock the pool, you should immediately use a wet vacuum method to clean up the areas around the pool. There is always a small chance that small traces of cal hypo will be left on the ground.

Avoid Breathing in the Dust

Cal hypo granules can be extremely hazardous when breathed in. This can irritate your nose and throat and cause shortness of breath. For this reason, I urge you to wear a respirator when working with cal hypo.

Frequently Asked Questions

Will calcium hypochlorite raise free chlorine in pool?

Absolutely! Cal hypo is a potent type of pool shock that you can use to raise the free chlorine in your pool. The primary use of calcium hypochlorite in pools is to raise and refresh the free chlorine levels.

How long after adding calcium hypochlorite can I swim?

I recommend waiting at least 8 hours before swimming after adding cal hypo shock to your pool. This is because it can take some time for the shock to dissipate and treat your pool. If you try to swim too early, the water can harm you or damage your swimsuit. Always test the water before jumping back into the pool!

How often do you shock your pool?

I advise shocking your pool around once per week during peak swimming season. If you are using your pool less than normal, this can be shifted to once every two weeks. However, weekly is always a good goal to shoot for to ensure your pool is nice and clean.

Cal Hypo Keeps Your Pool Clean

If you have a traditional chlorine pool, you will probably use cal hypo for shocking. This is one of the most important chemicals in a pool, as it helps keep your water clean and balanced. Knowing how to use cal hypo effectively and understanding its importance will give you a leg up in the constant battle of keeping your pool clean!

Reach out to me if you have more questions on how to use calcium hypochlorite. I am always happy to help a fellow pool owner!

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