Using Calcium Hypochlorite to Shock Your Pool – Cal Hypo Shock

Michael Keenan


Pool Water Chemistry

Do you want your swimming pool water to be free of bacteria and algae? Then you need to know about calcium hypochlorite shock!

In this article, I’ll explain what calcium hypochlorite is, its uses, how it functions, when is the best time to shock your pool with cal hypo, and why it is important for maintaining optimal pool health.

So if you’re looking for an effective way to sanitize your swimming pool without breaking the bank – read on!

calcium hypochlorite shock

Quick Answer:

Calcium hypochlorite is a great swimming pool shock treatment, commonly used in pool maintenance. Calcium hypochlorite is an inorganic compound that has been around for centuries, acting as a powerful disinfecting agent.

You should shock your pool at least once a week, especially after heavy use or bad weather, to maintain free chlorine levels and destroy chloramines which can cause skin irritation and respiratory problems. Be sure to use safety gear when handling cal hypo, then follow manufacturer instructions on how much to use and let it work for 8 hours before swimming again!

What Is Calcium Hypochlorite?

Calcium hypochlorite is an inorganic compound produced by treating lime with chlorine gas. It is also known as cal hypo. Bleaching powder containing cal hypo was the main disinfecting agent during the first world war, at that time it was known as chloride of lime.

It is a chemical that is highly alkaline which means that it has a very high pH level. Calcium hypochlorite has greater available chlorine than sodium hypochlorite which is found in liquid bleach.

It is the active ingredient in a lot of commercial cleaning products like bleaching powder, chlorine powder and chlorinated lime.

It is a white solid substance that comes in two forms – anhydrous and hydrous:

  • Anhydrous calcium hypochlorite is the dry form of the compound, this means that water content has been removed from this compound.
  • Hydrous calcium hypochlorite is the hydrated form with water being one of its constituent elements.

Cal hypo is not soluble in hard water, for maximum effect you need to mix cal hypo in soft or medium-hard water. It is a very strong oxidizing agent and reacts strongly with organic compounds, therefore, it should be stored very carefully in a dry area.

calcium hypochlorite powder form

Origins of Calcium Hypochlorite

Calcium hypochlorite is the active ingredient in bleaching powder. Bleaching powder was developed in the late 18th century by two Scottish chemists, Charles Tennant and Charles Macintosh.

They patented the process of making bleaching powder in 1799.

During the First World War, bleaching powder was used to disinfect trenches, dugouts and latrines. At that time it was known as chloride of lime.

Chloride of lime solution was the most widely used antiseptic solution in the First World War. This antiseptic solution was developed at the University of Edinburgh.

Uses of Calcium Hypochlorite

Calcium hypochlorite is a good disinfecting agent that has a lot of uses like:

  • Water treatment
  • Drinking water disinfectant
  • Bleaching agent
  • Swimming pool sanitizer

How Calcium Hypochlorite Works

Calcium hypochlorite is a good disinfecting agent because it contains chlorine. The pool shock products that you buy contains around 65% to 75% chlorine.

Chlorine binds with microbes and organic contaminants. When this binding occurs it alters the cell structure of these organisms and affects their ability to survive in the pool water.

Cal hypo will release around 0.8 parts per million (ppm) of calcium for every 1 ppm of free chlorine in your pool. High levels of calcium can cause scaling and reduce the longevity of pool equipment.

So it is important to test the levels of calcium in your pool water before you use a cal hypo shock to sanitize it.

The Best Time to Shock Your Pool with Cal Hypo

Calcium hypochlorite contains unstabilized chlorine. This means that it will burn away quickly if exposed to sunlight. It will not be able to remain in your pool long enough to disinfect it properly.

Therefore, calcium hypochlorite works best at night. You can add your cal hypo shock to the pool at dusk and leave it in there overnight. This will ensure that you wake up to a clean pool in the morning.

You can also use cal hypo shock with a stabilizer such as cyanuric acid. You’ve guessed it – it helps stabilze the chlorine and make it more effective.

night time with moon and a bird

Why You Need Calcium Hypochlorite Shock in Your Pool

If your free chlorine levels have fallen below the required quantity, a calcium hypochlorite shock helps you reactivate these sanitizers. They free up the combined chlorine atoms and make them viable again.

Introducing more chlorine into your pool’s water composition also boosts the sanitizer levels.

In the same way that cal hypo frees up combined chlorine, it also destroys chloramines. When chlorine reacts with bodily fluids and dead skin cells in the pool water it forms chloramines.

Chloramines are also the gas that causes the pool smell you usually associate with chlorine.

Chloramines can cause skin irritations and respiratory problems if not taken care of immediately. Calcium hypochlorite is an effective agent for the removal of chloramines from your pool.

How Often Should You Shock Your Pool

Shocking your pool will protect it against algae, bacteria and other contaminants. Ideally, you should shock your swimming pool at least once a week.

If you have a saltwater system in your pool you still need to shock it – just less often. See my guides on converting a pool to salt water and then my maintenance guide for salt water pools.

You should not let your chloramine levels become unmanageable. It is always best to maintain a regular cleaning and shocking schedule for optimal pool health.

You might need to shock your pool more frequently in some cases.

A bad storm can mess up the chemical composition of your pool. It might also blow dirt and debris into your pool. So always shock your pool after a storm.

It is a good practice to shock your pool after it has had some heavy use. Pool parties can leave oils, sunscreens and bodily fluids floating around in your pool water. Not only does this lead to excessive chloramine production but it also provides fertile ground for algal and bacterial growth.

A drastic change in the water level of your pool also warrants a pool shock. You will also need to shock your pool if any bathroom-related accidents have taken place in the pool.

What You Need to Start a Calcium Hypochlorite Pool Shock

Apart from the calcium hypochlorite, you need to equip yourself with safety gear before digging into this powerful chemical.

You will need to get your hands on the following equipment before you can start shocking your pool:

  • 5-gallon bucket
  • Safety goggles
  • Closed-toed shoes
  • Long-sleeve shirt
  • Chemical-resistant gloves
  • Wooden stir stick
calcium hypochlorite put in to shock pool

How Much Calcium Hypochlorite to Use

The amount of calcium hypochlorite you need to shock your pool depends on the size of your pool and level of water contamination.

The first step to finding out the right quantity of cal hypo is to test the water.

You need to identify the free chlorine (FC), combined or captured chlorine (CC), and total chlorine (TC) levels using a testing kit to figure out how much calcium hypochlorite to use.

Free chlorine is the amount of chlorine that is still available in your pool water that can effectively sanitize the water. When free chlorine reacts with contaminants to sanitize the water, it becomes combined chlorine.

The sanitizing capabilities of combined chlorine have been used up.

Total chlorine is the sum of the free and combined chlorine in your pool. You need to ensure that free chlorine levels in your pool are maintained to ensure that your pool remains healthy and clean.

Once you have identified how much the chlorine level should be raised, you can follow the manufacturer’s instructions on the packaging. There will be a chart or guide to help you do the calculations.

cal hypo swimming pool

How to Shock Your Pool with Calcium Hypochlorite

Once you have gotten your hand on the calcium hypochlorite, safety gear and the amount of pool shock to be used, you can dive right into the process of shocking your pool.

Fill three-quarters of the bucket with warm water and add calcium hypochlorite to the bucket one pound at a time. Stir it in slowly and ensure that the compound completely dissolves in the water.

Do not mix cal hypo directly into your pool water. It is a very strong chemical and can damage the pool’s surface. Distribute the dissolved mixture evenly into the pool. Walk around the perimeter of your pool and ensure that every part is covered.

If there is any residue leftover, I just scoop some pool water into the bucket, mix it properly and pour it back into the pool.

You should let the shock work for at least 8 hours. It is advised that you do not use the pool during this time as the high concentration of chlorine can cause skin and eye irritations.

The best thing would be to start shocking your pool at dusk and leave the cal hypo undisturbed overnight.

Cal hypo is a very strong chemical and can cause irritations if it gets into your eyes or on your skin. Make sure to clean all your clothes immediately after you finish shocking the pool. Keep a bucket of clean water handy in case it gets into your eyes while shocking.

Finally, clean the area around the pool so that there are no traces of cal hypo left that people might accidentally get into.


How often should I shock my pool with calcium hypochlorite?

You should shock your pool with calcium hypochlorite at least once a week and again after heavy rain or heavy use.

What other chemicals do I need to add when shocking my pool with calcium hypochlorite?

When shocking your pool with calcium hypochlorite, you should also add alkalinity, pH balancer, and stabilizer chemicals for the best results.

How long does it take for calcium hypochlorite to work in a swimming pool?

Calcium hypochlorite typically takes about 8 hours to work in a swimming pool after it has been added.

What are the safety precautions when using calcium hypochlorite to shock a pool?

It’s important to take safety precautions when using calcium hypochlorite to shock a pool such as wearing gloves and safety glasses and avoiding inhalation of the chemical’s fumes while mixing it into the water.

In Conclusion

Shocking your pool with calcium hypochlorite helps protect against bacteria, algae, chloramines, and other contaminants so you can enjoy a clean swimming experience. This powerful chemical should be handled with care as it can cause skin or eye irritations if not used correctly.

Make sure to wear safety gear while handling cal hypo shocks, follow all the manufacturer’s instructions carefully.

Before starting a cal hypo shock, test the levels of chlorine in your pool then calculate how much shock to use.

I like to shock pools at least once a week and more – after heavy usage or bad storms for best results. With calcium hypochlorite shock it’s best to let it sit overnight so you can wake up to a clean swimming experience!

Calcium hypochlorite is one of the best chemicals in the market that help you keep your pool sanitized and clean without breaking your bank! Give it a go next time you need to shock your pool.

michael keenan author

Author - Michael Keenan

I'm Michael Keenan the owner and creator of the Outdoor Care Guide. I'm a trained horticulturist with over 30 years of experience in pool care, plant care, and landscape care! Seemed like a good idea to share - I think I can make your life easier and save you some time and money!