It doesn’t matter whether you’re a new pool owner or an old hand, shocking your swimming pool might seem a little scary. That’s okay—handling such a high volume of chemicals can make anyone feel jittery. It gets easier with practice and knowledge.
Once you learn what pool shock is, understand why it’s one of the most important chemicals to use, and learn how to shock a pool, the whole process will start to feel pretty routine. That’s good, because shocking your pool on a regular basis is a simple way to prevent stray algae and bacteria from taking root in your water. It also helps keep your pool smelling … well, hopefully like nothing at all.
What Is Pool Shock, Anyway?
Think back to the last time you walked past a hotel pool. The chemical smell probably knocked your socks off, right? You may think that unmistakable smell is chlorine, but the odor actually comes from chloramines, a sign of improperly balanced water.
Chloramines form when the chlorine in your pool mixes with the nitrogen in sweat, oils, and urine (which we’re sure isn’t in your water, though we can’t vouch for the hotel). This is a natural chemical process, basically a byproduct of your chlorine doing its job.
In addition to giving your pool a funky smell, too many chloramines can also irritate your skin, eyes, and respiratory system.
By shocking your pool, you’re adding enough chlorine (or other chemical) to clean the water and destroy the buildup of chloramines. This process is also called superchlorination.
Pool Chlorine 101
Before we dive into how to shock a pool, you first have to understand the difference between total and free chlorine, and what combined chlorine and breakpoint chlorination are.
Free Chlorine (FC) is the amount of chlorine actively disinfecting your water. You want your water’s FC level to be between 1 and 3 parts per million (ppm) so the chemical can do its job.
Combined Chlorine (CC) is the chlorine that’s been used. It’s still in the water, but its sanitizing power is greatly diminished. You want to keep your CC level at less than 0.2 ppm.
Total Chlorine (TC) is the sum of FC and CC in your pool.
Pool water testing kits can measure the FC and TC of your water. To find the CC of your pool, simply subtract the FC from your TC.
Breakpoint Chlorination is when you have enough FC to shatter the molecular bonds of chloramine. You need to add ten times the amount of CC to hit this point.
Try to reach the breakpoint every time you shock your pool. Not hitting the breakpoint can result in even more chloramines in your pool, and if the chloramine levels continue to rise unchecked, you may eventually have to partially or even fully replace your water to fix the issue.
Types of Swimming Pool Shock
You generally can’t shock your pool using your regular chlorine tablets, but you do have your choice of products when it comes to pumping up your chlorine levels.
Calcium Hypochlorite
Also known as cal hypo, this chemical has been used to disinfect swimming pools and municipal water sources since 1928. It’s one of the most inexpensive and convenient ways to shock your pool.
Features
- Most commercial versions contain between 65% and 75% chlorine.
- Calcium hypochlorite needs to be dissolved before you add it to your pool.
- It must be used after dusk.
- It will be roughly eight hours before you can safely swim again.
- It adds about 0.8 ppm of calcium to your water for every ppm of FC added, so use caution if your water source already has a high level of calcium.
Lithium Hypochlorite
If your water has high calcium content, and you don’t mind paying a little extra, lithium hypochlorite is the way to go. It dissolves much more quickly than calcium hypochlorite, so you can add it directly to your pool without dissolving it beforehand.
Features
- Commercial versions contain 35% chlorine.
- It must be used after dusk.
- It will be roughly eight hours before you can safely swim again.
- It can be toxic to aquatic life (making it a solid algaecide), so you must take care when disposing of recently treated water.
Note: You may have a difficult time finding lithium hypochlorite. Some pool chemical manufacturers have stopped producing it due to the rising cost of lithium, most of which is now used to make lithium batteries.
Dichlor
The actual names of this swimming pool shock chlorine are sodium dichloro-s-triazinetrione or dichloroisocyanuric acid. (Try saying either of those five times fast.) Dichlor shock is much easier to say and even easier to use. Depending on the brand, you may be able to add it directly to your pool.
Features
- It typically contains between 50% and 60% chlorine.
- You can use it for both regular chlorine doses and shock treatments.
- You typically don’t have to dissolve it ahead of time.
- It adds 0.9 ppm of cyanuric acid for every ppm of additional FC.
- It must be used after dusk.
- It will be roughly eight hours before you can safely swim again.
Non-Chlorine Shock
If you’re looking to shock your pool and take a dip soon afterward, this is exactly what you need. Non-chlorine shock using potassium peroxymonosulfate is a fast, inexpensive pool shock alternative.
Features
- You can add it directly to your pool water at any time.
- It will be roughly 15 minutes before you can safely swim again.
- Because it doesn’t rely on chlorine, it does not function as an algaecide.
When to Shock Your Pool
You might have noticed the instructions on chlorine shocks say they need to be used after the sun has gone down. This is because the sun will burn off unstabilized chlorine, which means the shock won’t be as effective. Shocking your pool at night makes sure the chemicals work the way they were meant to.
How Often to Shock Your Pool
You don’t want to wait for a bad smell or itchy eyes to happen before you shock your pool. We recommend shocking your pool once a week, or at least once every other week to properly maintain your water chemistry. The more often you use the pool, the more often you should reach for the swimming pool shock.
In addition to your weekly or semi-weekly treatments, you may want to perform an extra pool shock under certain circumstances, such as after:
- heavy pool use (like a pool party)
- a severe rainstorm or damaging winds (especially if your pool collected debris)
- a major water level change
- a bowel-related pool accident
Think of extra shocks as insurance against wayward algae and other contaminants. It’s better to take out any bacteria before it has a chance to affect the quality of your water or make anyone sick.
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How to Shock a Pool
You’ve got the chemical knowledge. Now it’s time for practical experience. Superchlorinating your pool is shockingly (sorry, we couldn’t resist) easy once you get the hang of it.
Important: Remember, if you’re using chlorinated pool shock, wait until the sun goes down before adding it.
Supplies
- Protective eyewear
- Chemical-resistant gloves
- Pants and long-sleeved shirt (that you won’t mind messing up)
- Closed-toe shoes
- Water test strips or liquid testing kit
- Shock of your choice
- 5-gallon bucket (if you need to dissolve the pool shock before adding it to the water)
- Wooden stick (for stirring to dissolve)
Before you start, you’ll need to calculate your pool’s volume. If you don’t already know how much water your pool holds, you can use this pool calculator to figure it out.
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Instructions
- Suit up in your protective gear.
- Do a quick round of pool water testing for FC and TC to determine how much CC is in your pool. This measurement will dictate how much swimming pool shock you need.
- Carefully read the manufacturer’s instructions on your shock product. Most packages include charts or steps to help you calculate how much shock you need. If you need to calculate it yourself:
- Subtract the FC from the TC to find the CC
- Multiply the CC by ten
- Subtract the FC from that sum
- Find how many ounces of shock will produce a 1 ppm chemical change in 10,000 gallons of water
- Divide your pool volume by 10,000 gallons
- Multiply the chemical change by the divided pool volume and the CC/FC difference
- Convert the answer into pounds by dividing by 16. The result will be how much pool shock you need to use
- If you need to dissolve the shock first, fill your bucket roughly ¾ full with warm water. Otherwise, skip to step six.
- Add the shock to the bucket, and slowly stir until the chemical is as dissolved as possible. Work in one-pound increments. Skip to step seven.
- If you do not need to dissolve your shock, count how many containers of shock you need. Add one bag at a time until you reach your calculated breakpoint.
- Slowly pour the shock while walking around your pool for more even distribution. (If your shock-water mix has solid particles at the bottom, dip the bucket into your pool water, carefully swish around to dissolve, and keep pouring.)
- Wait to use your pool based on when you added the shock and the manufacturer’s recommendations. You don’t want to irritate your skin and eyes or bleach your swimsuit.
Pool Shock Safety
Shocking your pool is a necessary part of good maintenance, but please remember those chemicals can be extremely dangerous if they’re mishandled. We’re not exaggerating the danger—improperly stored chlorine can literally explode.
Always wear protective gear—especially goggles and chemical-resistant gloves—when handling and dissolving chlorine. Pool shock, especially calcium hypochlorite, can sometimes release small amounts of chlorine gas. Wearing protective gear can help prevent eye and skin irritation.
Do your best to avoid directly breathing from the containers. Exposure to chlorine gas can cause throat and lung irritation. You probably don’t need a dust mask, just try not to breathe too close to the container. If it makes you feel safer, though, you can use a chemical mask.
Never, ever mix types of pool shock. We don’t want you to become a home science experiment gone wrong. Mixing liquid chlorine or even dry chlorine granules can cause a volatile reaction. Add each chemical to your pool separately.
Do not add shock directly to the pool water unless instructed to do so. If the package says dissolve before adding, make sure to do just that.
Only open one container at a time. If you need to use more than one container of shock, make sure you completely empty out each container before moving on to the next one.
Don’t be Shocked, You Made It!
Now you know why, when, and how to shock a pool. There might be a little math involved, but aren’t occasional brain teasers good for your memory anyway? We’re pretty sure that’s true, but we may have forgotten.
Anyway, however you feel about the math, regularly using pool shock can prevent more work in the long run. It’s one of the easiest ways to keep bacteria at bay, your water clear, and your pool time fun.
Happy Swimming!
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