The problem with water purification is that it’s impossible to know if water is tainted and dangerous to drink until it’s too late! On top of that, there are many ways to purify water, and knowing which method is best can get confusing. One popular method is using pool shock, and you’ll find this recommended in many survival forums and websites. However, caution must be used! Calcium hypochlorite is tricky to store safely, calculate the correct concentration, and know how much to purify water.
Here’s what you should know from a water safety expert.
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This article updated in April, 2026 with current EPA guidance.
Expert advice for using calcium hypochlorite
When I was writing my book Survival Mom, I was a stickler for accuracy. Chapter 2 is all about water storage and purification, so I went to water expert Alan Martindale, the Water Quality Supervisor for the City of Mesa in Arizona, with all my unanswered questions about water safety.
I asked Alan about using calcium hypochlorite, aka pool shock, to purify water because I wanted an expert to weigh in on this popular survival and prepper topic. I’ve written directions for its use here.
Here is what Alan had to say:
“Lisa, I would agree that calcium hypochlorite can be used to disinfect water. As you mention, the key is to make sure it is intended to be used for purification of drinking water. The standard for this is National Sanitary Foundation (NSF) approved. Calcium hypochlorite can be purchased in several concentrations. Although 65% is most common, you will also see 78% available chlorine.
Need to be real careful with chlorine concentrations. You shouldn’t really drink much over 2ppm (parts per million) or it can cause diarrhea. The maximum contaminant level (MCL) set by the EPA for chlorine is 4ppm.
A huge issue associated with chlorinating is the “chlorine demand” of the water being treated. If one water source has more contamination than another, it will take more chlorine to make it safe. Could be two or three times more. Many emergency sites recommend having some test equipment to verify chlorine residuals. A pool test kit or test strips will work just fine.
Calcium hypochlorite will work great but, it will take more experience and care to get safe results than using unscented laundry bleach.”
Updated EPA Guidelines (2026)
The EPA’s current guidance says to add one heaping teaspoon (approximately ¼ ounce) of HTH to two gallons of water, stir until dissolved, then add one part of that chlorine solution to 100 parts of water — roughly 1 pint of solution to 12.5 gallons of water.
If the chlorine taste is too strong after treating, pour the water between clean containers and let it stand a few hours before drinking.
Calculate the right levels of concentration
Pool test kits are inexpensive and can be purchased just about anywhere pool chemicals are sold. I recommend that everyone who owns calcium hypochlorite and is planning to use it to purify water have at least one pool test kit on hand. That will be the best way we amateurs can determine whether the chlorine level of our purified water is safe to drink.
Alan states that using store-bought bleach is easier (8 drops per 1 gallon of water for purification purposes), but bleach has a shelf life. It begins to lose effectiveness after several months, thus the popularity of calcium hypochlorite in survival circles.
A much easier method to use
Ultimately, I do not recommend calcium hypochlorite for water purification. Storage can be tricky, and as Alan describes above, care has to be taken in order to calibrate it correctly.
Calcium hypochlorite should be stored in a dry, well-ventilated area at a temperature below 120°F, separated from acids, ammonia, amines, and other oxidizing agents, and while it’s not flammable, it can react explosively with ammonia, amines, or organic sulfides. When stored properly, it can have a shelf life of up to two years.
There are far easier methods. A Boroux water filter, for example, will filter thousands of gallons of water safely without having to take a crash course in amateur chemistry.
Another product on the market that combines the efficacy of calcium hypochlorite with the ease of using bleach is P&G Purifier of Water Portable Packets. These packets come in a set of 12, are small and portable, and each quickly purifies up to 2.5 gallons of water.
These packets are perfect as a backup in bug-out bags, camping/backpacking supplies, or to have on hand for an emergency if a boil notice is issued and you need pure water quickly. They take all the guesswork out of purifying water. Also, you don’t have the issues of possible flammability and storage as you do with pool shock packets. I recommend this product as a handy backup for purifying water.
When it comes to water safety and your family’s survival, you want to always rely on the most effective methods and have at least one or two backup methods as well. Calcium hypochlorite has its place among your emergency preps — just be aware of the learning curve and know what you’re doing before a crisis hits.
Last week, I read an article that discusses some of the dangers of Calcium Hypochlorite. I don’t remember exactly where it was, but Prepperwebsite was the path to it. It was said that one must take care in the storage method, because it will eventually deteriorate the container, unless all glass. Also, when in a sealed container, the decomposition generates chlorine gas. If one does not take care when opening the container, breathing the fumes would become quite an immediate health problem. Perhaps some cautionary disclaimers are in order?
I’m a sales rep for a large specialty chemical company that for years have sold Cal Hypo in both solid pucks and granular for pool sanitizing. Most states require 1 ppm as the lower threshold for pools and 5ppm on the high side. The problem is all forms of chloring degrade with time, so exactly how much to use in any “recipe” can change depending on how old the cal hypo is. What I want to find out is to what level (in ppm) do we need to achieve for proper bacterial kill? That is the question we should all need to know. If it’s X-ppm, then so be it. Even if the level is 50ppm, one can prep drums of contaminated water, killing off the bugs, then park it as the chlorine will gas off in a short time, faster in sunlight. But the question is how much to we mix to? 5ppm, 50ppm? If a pool operator has a swimmer that either vomits in the pool, or a child with diarrhea, the pool has to be evacuated and super chlorinated to at least 25 ppm for 24 continous hours. So is that the threshold for untreated water? I don’t know, but don’t want to risk getting very sick by under treating the questionable water. Perhaps Alan could shed more specific light on this…
Tim, this is information from my book, per Alan:”The safest level of chlorine in drinking water is around 2 ppm.”
I don’t believe you’ve answered Tim’s question.
I’ve tried to get in touch with Alan and haven’t heard back. I’ll check with the City of Mesa to see if he still works there.
Can’t a person use the chlorine concentration to make a potent bleach solution and than better manage the amount of product to treat smaller water supplies. For example, make into solution and than just use drops to purify a gallon or two at a time?
I am a water operator in a chemical plant, the EPA and each state has the required minimum chlorine residuals on their web sites. In my state the bare minimum for free chlorine residual in distribution is .2 ppm. Residual free chlorine is what is leftover after the demand is met. So depending on the condition of the water you are treating, depends on your dosage. Checking for the remaining residual is the safest way to ensure your drinking water Residuals. FYI our maximum residual is 4.0 ppm free chlorine residual. I would, if i were forced to treat water at home, personally like my residual somewhere in between the two say, 1.0 – 1.5 ppm. Water treatment is more complicated than applying chlorine. Some nasty critters are chlorine resistant, and must be filtered. The water that comes from your tap, is monitored and treated with very strict restrictions, by licensed professionals, just saying.
Thank you so much for sharing your expertise! My usual source for this information, Alan Martindale, has possibly retired from his position because I haven’t been able to get in touch with him. I appreciate your input.
You are so welcome. I just finished your book, I loved it, read it in a day and a half. I couldn’t put it down. It was refreshing to read it from a common sense perspective, since the topic is seemingly overwhelming. Looking forward to the next one!
Thanks, Water Operator. I completely agree. Deciding on the best way to treat water depends on at least two things: (1) testing it to determine what’s in it and at what concentrations so you can know what you need to treat for, and (2) knowing what the effluent’s use is and what criteria it needs to meet for that purpose.
There are some decent old-timey methods that work well for a lot of home uses, especially as a pre-treatment. My favorite comes out of a free book called “Household Discoveries”. The idea is kind of like a slow sand filter, but it has multiple layers: some kind of fabric on top of the barrel for course filtration, and layers of charcoal, sand, and pebbles. After this you could disinfect with bleach, UV, or something similar.
I know chlorine is utilized for water treatment. Not only in swimming pools but even on water used for drinking and other personal purposes. We really need to know how water treatment works using certain chemicals, this one is a good source.
It’s a good thing you discussed important points about the use of calcium hypochlorite for water purification. Just the right kind of information everyone needed to know. I hope you come up with more of this kind of posts. We all know how chlorine works and where it’s utilized. It’s important that we know the pros and cons about it.
It most certainly is hard to follow, especially when you need to vary your quantities of chlorine based on how badly contaminated the water is. It would seem a difficult task to figure out what is more contaminated than another.
So, moving right along, how does iodine fare in the scheme of purification vs chlorine? Is it more reliable, or is it less reliable than chlorine?
Thanks
I’m not a big fan of using iodine because it comes with so many warnings for its use. The Red Cross uses a combination of boiling water AND using bleach, so in scenarios in which water is significantly contaminated, that is my suggestion as well.
Send me inf.cal hipo
I just clicked the ‘here’ link to get 100% calcium hypochlorite. the only problem is, they have SEVERAL items listed on that particular page and I have NO idea WHICH one is the 100% pure stuff. Can you help?
Thanks!
Hi, I was reading all the comments about the calcium hypochlorite and the problem with the proper amount to use. One thing that I have stored up on is brita filters for this very reason. From my understanding the brita filter does filter out the chlorine. I plan to use the calcium hypochlorite first and then filter through a brita filter just to be sure I get at least most of the chlorine out. I also have the Katadyn Ceradyn Water Microfilter but it does not filter chlorine, thus the brita.
I did find an article a long time ago, this is what it said regarding the amount to use.
Using granular calcium hypochlorite to disinfect water is a two step process.
• To make a stock of chlorine solution (do not drink this!) dissolve 1 heaping teaspoon (about one-quarter of an ounce) of high-test (78%) granular calcium hypochlorite for each two gallons (eight liters) of water.
• To disinfect water add one part of the chlorine solution to 100 parts water to be treated.
• Let the mixture sit for at least one-half hour before drinking.
I would not rely on the Britta water filter ; I would use ZERO water filter(Much better) ; check results @ Healthranger , not sure of website , but just Google Health Ranger & you can find it ; You really need to see the TEST Results ! ZERO water Filter is the BEST you can buy in the “WORLD” as far as I can ascertain ; BEST water is “DISTILLED” , I have been using Distilled for over 40YRS. , I’am CLEAN !
For those of us not on chlorinated water….chlorine IS the problem, when we go to the city. ..It galls me to have to buy water bottled in drought zones, to have descent water to drink….. The Zero water system would be the same thing, in that, it removes all minerals as well….as the chlorine… and I Want the minerals in my water, at least the few I have in mine…need them to help keep my my levels up…
I agree about the minerals. They’re important. Would distillation work for you?
I cant find calcium hypochlorite that im sure has no algaesides,or clairifiers. also What is the highest concentration available? Where can I find it online?
You can get 99% on Amazon
Can you please post some links to what you believe is safe? I see so much out there but when the other chemicals are not specified, it concerns me greatly. Thank you!!! I’ve looked and cannot find ANYTHING that is 99% pure.
I appreciate the above discussion regarding ppm of chlorine to treat water for disinfection, and residual amounts of Cl for safe water drinking. What is missing from this discussion (and perhaps is covered elsewhere – I’m new to this discussion) is any description of the mechanics of actually treating water with calcium-hypochlorite crystals. For the sake of discussion, let’s say I have a 5 gal container of water at 50 F, and I treat it with 3 “period” sized grains of 99% Ca-hypochlorite (which I have on hand). I don’t want to waste precious fuel by heating the water. So, how long to I need to wait until I can consider the water “disinfected”? While agitation may help in getting the Cl into solution with the water, this can be a slow process.
The trick is in releasing the Cl from the Ca-hypochlorite so that it can then disinfect the water. Heat is an obvious agent which can facilitate in releasing the Cl from the Ca-hypochlorite. Other than heating the whole 5 gallons (a huge waste of fuel), is there some way to make a small mixture of Ca-hypochlorite and water, heat the mixture (under pressure, so that the Cl gas does not escape), and then then release the mixture into the 5-gallon water container? Obviously “yes”, but it requires a pressure vessel provided with an over-pressure release valve (to ensure that the pressure vessel does not rupture during heating). Complicated plumbing, indeed!
Would it not be better to perhaps obtain a pressurized cylinder of chlorine gas, and then introduce the gas into a 5-gallon container of water (with a pressure release valve on the container set at something like 5 psig – or whatever the container can withstand)? After gassing the container of water with the chlorine gas and then letting it set (pressurized) for say 12 hours, one could then vent the container using a check valve (to let Cl out, but not pathogen-bearing air in). At that point one can then test the water for residual Cl content. If the Cl content is too high, put the container in the sun and let the solar heat drive off excess Cl.
Of course the best way to purify water is by boiling, but I think we’re assuming that fuel will be a precious commodity, and using fuel to boil water is a waste of a precious resource when other methods can be used to purify water – such as using Ca-hypochlorite crystals. So, let’s hear from someone about the actual mechanics of using Ca-hypochlorite for water purification – i.e., HOW do I use to use Ca-hypochlorite to purify my water – or, more specifically, how do I best release the Cl from Ca-hypochlorite for water purification?
Calcium hydrochloride is highly volatile it will explode if exposed to heat or flame or sunlight oar even moisture look up the data again it’s highly volatile quite a few things can make it explode or catch on fire be very careful using it and storing it. Research research research
I did some MATH!
I calculated that this recipe makes solution at 508ppm and the final product at 5ppm.
Here is my math, and know my math is better than my spelling.
Solution is
1tsp shock: 2 gallons water
(Fun fact 2 gallons of water is 1,536tsp)
So the solution is 1part shock:1536 parts water.
(Divide a million by 1536)
Solution is 651 ppm of SHOCK to water.
But what persentage of calcium hypochlorite is in your shock? Multiply that persentage(as a decimal) to 651. Here we have 78% or .78 so 651*.78 is about 508ppm.
Now for what we drink
1 part of 508ppm solution to 100 parts of 0ppm stream water
(1*508+100*0)/101=5ppm.
Now the expert said “You shouldn’t really drink much over 2ppm (parts per million) or it can cause diarrhea. The maximum contaminant level (MCL) set by the EPA for chlorine is 4ppm.”
But, now what about decay?
Havent found soild decay rates yet, but it looks like air and heat help decay. Id imagine 30 minutes in a sealed container in a cool place gives it time to do its job, then in the sun in an open container or pouring it back and forth from container to container or airorationg it by other means could bring it down. But what is the best way to break it down?
What is the “chlorine demand” of the water?
I am thinking I will filter with Sawyer Mini and use this solution at half strength to kill any viruses. Then let it sit much more that 30 min so it can clean and dissipate.
IF I have tank with capacity of 13500 M3 water.I WANT TO use calciume hypochlorite 75% concentration.what is the amount of calciume hypochlorite can I added to tank to get the chlorine in the rang 0.5–3ppm.what is the method that I use to calculation?
Your link in the linked pdf ( https://thesurvivalmom.com/wp-content/uploads/2010/08/Using-Pool-Shock-as-a-Water-Purifier.pdf ) for inyopools.com as a source for 100% calcium hypochlorite should be deleted as they do not sell 100%. (http://www.inyopools.com/ProductListing/chemicals_shock.aspx?Keywords=shock). They offer 68 and 73%.
Hello could you please tell me the contact time period of calcium hypochlorite (TCL). If i add small amount of TCL, who much time required to totally lost TCL ?
how much calcium hypochlorite would you need to disinfect a 5 gallon HDPE plastic water tank?
Thank you. Our well was contaminated with coliform bacteria. I do pet rescues and lost several cats before I figured out it was our water. We hired a professional but before that, and now so I can retest water. I needed safe water for my pets. Veterinarians couldn’t figure it out. Thank you so much.
I have read several articles about this also
Deb, where do you get yours from? What brand is it? Thanks
Anyone consider Chlorine Dioxide Solution as an alternative to Bleach?
It can be made on the fly (over 12hrs) with 2 ingredients. (sodium chlorite & citric acid)
Some cities use it to purify water.
Dr Andreas Kalcker has a lot of info on making CDS.
Here is a good beginner video about cds. His focus is internal use but you can use it for external use too.
https://rumble.com/v5w6uj8-cds-for-beginners-wdr.h.c.-andreas-kalcker.html
Safrax sells ready to go cd tablets for water purification too but are a bit pricey.
https://safrax.com/shop/
But like someone said: Research first.
Better yet, keep it simple. Get a high quality water filter such as Boroux, add the flouride filters, and those will last many thousands of gallons. As a backup, the P&G water purification packets are simple, inexpensive, and don’t require any special storage or calibration.
Yes, other methods are possible, but why when there are other methods just as effective and far more simple. Even boiling is more simple.