Pool Shock: When & How Much?

Dr. Pool of In The SwimWelcome to our discussion on swimming pool shock – specifically, when should one shock the pool, and how much do I need to add?

This question is emailed to me quite often, so I thought it deserved an answer in print.

There are 3 main reasons to shock a swimming pool:

1). Remove chloramine bonds
2). Kill visible algae
3). Kill invisible pathogens

1). Remove Chloramine Bonds

Chloramine Molecule - cool, huh? Free Available Chlorine (FAC) is an attractive target for ammonia and nitrogen that gets into the pool water. When combined, FAC becomes ineffective as a sanitizer and causes eye irritation, or red eyes.

Although it seems counter intuitive, when your pool smells strongly of chlorine, chloramine bonds are likely at a high level, and a good pool shocking is indicated. There is another way to determine chloramine levels, however.

Using a DPD test kit will allow you to measure both FAC and Total Available Chlorine (TAC). Subtraction of Free from Total will yield the amount of Combined Available Chlorine (CAC), commonly known as chloramines. When a level of 0.3 ppm is reached, a good pool shocking is indicated.

Shocking the pool to a level that is 100x greater than your chloramine is effective at chloramine removal. For a level at 0.3 ppm CAC, add enough chlorine to reach a level of 30 ppm in the pool.

2). Kill Visible Algae

Algae close-up under the microscropeChlorine is a great algaecide, I like to say. Pool shock, in the right amounts, will destroy all types of algae by slashing through their slimy outer shells and disrupting their cellular processes. Pink, Green, Yellow – none is a match for  chlorine, in the right amounts.

Shocking the pool to 30 ppm of free chlorine residual, in the presence of proper pH levels, will destroy most algae. For best results, the pool should be vacuumed before shocking and brushed after shocking. Filter non-stop, backwashing as needed, and use a pool clarifier if needed.

3). Kill Invisible Pathogens

Pool bacteria is blue, right?A pathogen is a disease-causing substance. Bacteria, Viruses and Parasites can live in pool and spa water. Your normal sanitation and filtration likely removes most pathogens, but if you want to be sure – a good pool shock would be in order.

Shocking the pool after heavy use, or an extended period of low or no chlorine, or if the water has not been filtered for some time – are all good reasons to shock the pool.

The amount of shock needed to remove pathogens is dependent upon the level of pathogens in the water. You can test for bacteria with a bacteria test kit, or you can just shock the pool to 30 ppm.

How Much Pool Shock Do I Need?

Breakpoint chlorination is a level of chlorine at which point molecular bonds are broken apart. Conveniently, this is also the point at which chloramines, algae and pathogens are removed from the water.

For most situations, a level of 30 ppm of chlorine will reach the threshold of breakpoint chlorination, and order will be restored.

Example: 20,000 gallon pool, use 7.8 lbs of Cal Hypo Shock

Example: 20,000 gallon pool – 7.8 lbs of Cal Hypo to reach 30 ppm

Here’s a pool shock treatment chart to determine the amount of pool shock necessary to raise the chlorine level above the breakpoint threshold, which is usually around 30 ppm.

Not using enough chlorine, to not quite reach breakpoint, may not have the desired full effect.

Check for proper water balance, especially pH, in the range of 7.2-7.6, before adding the shock to the pool. High pH of 7.8-8.2 can render up to half of your pool shock completely inactive.

Proper filtration and circulation deserve some mention as well – pool shock doesn’t do it all alone. The pool water needs to be filtered to remove the microscopic remnants of organic matter that is destroyed.

Keep those questions coming!



Dr. Pool



Pool Shock: When & How Much? — 30 Comments

  1. I’ve got a 36,000 gallon saltwater pool. The deep end has a light green tint to it. The shallow ends look crystal clear. Walls feel a little slimy but have felt that way since pool was installed (approx. 3 weeks ago). I’ve got a DE filter along with a chlorinator. What is the proper procedure to shock and treat?

    500ppm hardness
    Total chlorine/bromine 0
    Free chlorine 0
    pH 7.6
    Total Alkalinty 240
    Cyaniric Acid 30-50

    How much pool shock would you use to get rid of green tint (I assume algae just starting). Thanks in advance!

    • Hi Steve, I would use 4 lbs of granular pool shock (Cal Hypo), for a pool in your condition (act fast). Check chlorine level 8 hours later or in the morning, and you should still test a high level. If it is zero 8 hrs later, you may need to shock again, with a heavier dose, to kill whatever is consuming the chlorine. shock works best at a low pH level, so if possible, lower pH to 7.2 before shocking, although your very high alkalinity level may resist changes to the pH. Liquid Acid or Dry Acid is used to lower both pH and alkalinity, so treat your alkalinity first, to lower below 150 ppm, by adding acid, repeatedly as needed, until alkalinity comes into range. Your pH level may go too low however, during alkalinity reduction, so you may have to add pH increaser, in between alkalinity treatments, to keep pH above 7.0.

      Shocking the pool adds chlorine to the pool just like tablets or your saltwater generator, the same hypochlorous acid, but it does it fast and all of a sudden, raising chlorine levels very high to kill algae, bacteria, and break apart contaminants and chloramines. It can also be used (granular chlorine) to maintain chlorine levels during equipment problems. And although many salt systems have a ‘shock pool’ feature, many owners don’t want to stress out the salt cell too much, plus it’s still much slower than pouring in chlorine powder, or using non-chlorine shock (useful for contaminants and chloramines, but not a disinfectant). Saltwater pool owners may use pool shock during pool opening and closing, or 1-2x per month to oxidize and sanitize the water, to reduce the work load for the salt cell, and to reduce chloramines, or treat for algae or cloudy water.

  2. Starting pool up from sitting over winter/spring (new chore for me). Size is aprx.11750 gallons ph, alk, both very low. from reading am adding around 7 lbs. sodium bicarb to raise alk and hopfully ph. after application how long do I wait before shocking pool and how much shock do I use for my pool size?
    Thank you.

    • Hi Beverly, only an hour or so, then recheck chemistry. For best potency from shock chlorine, a low 7.2-ish pH is best, so if it’s still a bit low that’s oK, you can adjust up to 7.4-7.6 after shocking. For your pool size, I would use 2 lbs of pool shock for hazy water, and 3-6 lbs if green, less if you can see the floor, more if not. If dark green, keep adding until water turns blue/gray color, with no hint of green, then brush and filter, then vacuum to waste.

  3. 30,000 gallon inground pool. Currently growing algae on textured walls. I am just starting the big shocking process to kill the algae while brushing. I am currently using bleach for chlorine to kill. How many gallons of bleach per hour should I add for this size pool? Thanks in advance.

    • If the algae bloom is small, 10 ppm should kill it. If the entire pool is green, go for 20 ppm, and if it’s dark green with low visibility of 12-24″, shoot for 30 ppm. For regular Clorox, store brand bleach of 6% strength, and based on poolcalculator.com – 10 ppm would be 622 oz, or double or triple the dose, to reach 20 or 30 ppm. Divide total amount by 128 oz (in each gallon).

  4. my pool is 26 around and I was wondering how much shock it I need to put in if I have green water?

    • Hi, just enough until it turns a blue/grey color, without any hint of green. Your pool has about 12000 gallons, so I would start with 4 lbs of pool shock (pre-dissolved in a bucket of water). If it still looks green-ish, add a little more. Be sure the pH is on the low side, 7.2-ish, before shocking. Add shock in the evening. Check pH and chlorine the next morning, and if the chlorine is zero (0), and the water looks greenish again, then hit it hard again.

  5. Hello i have a 16×48 inch pool. I just set it up yesterday and was wondering about the cleaning process. I bought super shock and chroline tablets. How much of each am i supposed to put and how often? Also how long do i wait to let my kids go in after the process?

    • Hi Lupe, Use 2 tablets in a chlorine floater, or enough to produce a consistent and constant 1-2 ppm of chlorine, as tested by your kit or strips. Replace them before they dissolve completely, which will be every 5-7 days probably. Use the shock chlorine once per week or twice per month, to super chlorinate, to kill anything that the tablets did not kill. Also can use the shock if the water gets cloudy,or if you see algae, or if you forget to fill the floater and the chlorine drops to zero. Always pre-dissolve the shock into a bucket of water before pouring into the pool. Check the label for wait time, but I would wait at least 4 hours. Also check and maintain the pH level in the 7.2-7.8 range, using pH adjustment chemicals if it drifts outside of this range. Your pool may also do well with regular additions of algaecide and clarifier. Finally, run your pump more than you think you need to – 12-24 hours per day, your filter is likely very small, and needs to run often, especially when it’s hot.

  6. How long should one wait before opening the pool after a shock treatment?

    • Hi, When using granular chlorine pool shock, there is generally a waiting period, consult the label for specific time. The main concern that undissolved granules can get in your eyes or ears or nose, or get stuck between your suit and skin and cause irritation. Some pool shocks like Lithium or Non-Chlorine shock are specifically “No-wait, swim immediately”, but Cal Hypo or Dichlor shock will instruct you to wait a certain period of time. If you really want to swim after shocking, a quick dip wouldn’t hurt, just keep heads above water, and don’t use a suit you like, which could become a bit faded.

  7. Okay so I have a 3,512 gal above ground pool and I poured half a bag of shock in my pool but I didn’t stir into a bucket, I just poured in my pool from the bag ? Is that bad ?? What should I do

    • Hi Lizzy, it’s okay – doing that once or twice per year won’t have much effect. The concern with vinyl pools is that undissolved shock granules can bleach the vinyl, or rob the vinyl of plasticizers and resins, or even burn through a very thin spot. However, this normally does not come from one careless mistake, it takes years of regular abuse, in most cases, to damage the vinyl. What concerns me more, is how you are storing your half empty bags of shock – please be careful to close them up very tightly with two paperclips, or a large chip clip, and then keep inside of a sealed container. Moisture, dirt, insects that get inside can cause hazards, or if granules spill from a loosely open bag, that could cause other calamity. Please be safe, store opened bags of shock very carefully, closed, clean and dry.

    • Hi Julie, brown water could be from metals, or silty dirt, or oaky leaf tannins. In most cases it will shock out of the water well. After balancing the pH to the low side, 7.2-ish, add enough shock to turn the pool a blue/grey color, with no hints of brown. Could take 2 lbs, or could take 4 lbs, add 1 lb at a time until the change is observed. Then add another pound to be sure. Run the filter all night and tomorrow you can add a clarifier if needed to help the filter.

  8. Hi i have a 3,300 gallon 14×42 pop up pool and i i just set it up but dont know how much shock to put at the beginning and when its green or when needed? Help please!!!

    • HI Maribel, Check the label on the shock, most will say to use 1 lb. per 10000 gallons of pool water. For your pool 1/3 lb will be a normal shock, used with fairly clear water conditions. For very hazy water, double the dose, for visible algae, triple the dose, to an entire pound, or the whole bag. Pre-dissolve first, by pouring into a 5-gal bucket pre-filled with water, and stir with a stick to dissolve. Add more water at the end, to dissolve remaining granules, and let sit for an hour, then pour the rest in the pool. Before using shock, make sure your pH is on the low side, 7.2-7.4 range. If using only a partial bag of shock, please be careful and close the bag up tightly, and clip with a chip clip or paper clip, to lock out moisture, dirt and insects, which could cause a reaction and possibly ignite or explode (really!). Keeping open, half-bags of shock laying around can be dangerous.

      • This is the exact dimensions of my pool as well, my question is, do you recommend having a chlorine tablet floater in there as well, after the shock? if so i have already purchased 3inch tablets, will they work? I am just now setting up a new pool. thanks for your help!

        • Hi Samantha, yes you need both. Shock is only for regular oxidation, or to kill algae, remove chloramines or correct other water conditions – but, you must have a daily chlorine method as well. Tablets are used to provide a constant and consistent 1.0-3.0 ppm level of chlorine in the pool. Add enough tablets to the floater, so that your testing results in a consistent 1-3 ppm, usually 2-3 tablets per 10000 gallons. When they dissolve, replace them promptly, so chlorine never goes to zero (never ever!). Use shock every few weeks to kill everything that the tablets miss, or after large parties, or for algae, or if you forget to refill the floater 😉

  9. I am new to this whole pool this. I have a 15,000 gallon pool that I have spent $300 on this week and it keeps getting worse. I put 4lbs of shock in it last night and it doesn’t even look like I put any. What can I do?

    • Hi Kayla, sorry to hear of your struggles. It can be confusing, this path to clear water, but you’ll get there!

      First thing is good water balance, proper pH, alkalinity, and calcium hardness and cyanuric acid (stabilizer levels). They should be 7.2-7.6, 80-120 ppm, 200-400 ppm and 20-50 ppm, respectively.

      Second thing is good filtration, if your filter is undersized or underperforming, it can really hamper algae clean-up efforts. Consider changing the sand or cartridge, or using a filter cleaner product to restore better filtration. And, be sure that filtration is occuring (you should be needing to clean the filter, or backwash). Missing internal parts or valve issues or other problems can cause water to bypass the filter media.

      Third thing is good circulation, at this stage you should run the filter nearly 24 hours per day, brushing and skimming daily, and running a pool cleaner if you have one, anything to help increase and improve circulation. Many people don’t run the filter long enough each day, and it’s a big part of the problem.

      Finally, heavy dose of chlorine – 4 lbs of shock for a 15000 gallon pool is a healthy dose, but if the chlorine reads 0 the following morning, that means that the chlorine demand was higher than the supply – it just ‘ate it up’. Making sure your pH is on the low side 7.2-ish, shock again in the evening, with 3-4 lbs of shock, brush and run the filter all night. In the morning you should have a chlorine reading. (if not, repeat)

      So, clear water is not just shock, but filtration and circulation is also very important. Keep at it, and let me know if you have other questions! And, keep reading the blog, we have lots of good information on water chemistry, algae killing, and other fun stuff!

    • Hi Joe, a normal shock is 1 lb per 10,000 gallons, so for your pool about 1/10 of a pound – but in the presence of algae or colored or cloudy water, a 3x or 4x shock dosage is not unusual. If your water is clear, I would use 1/4 cup, or 4 oz of shock (1/4 bag). If there is algae or cloudy water, use 1/2 cup or 1/2 bag, or more if needed. Pre-dissolve by pouring into a 5 gal bucket of water, and stirring to dissolve, then pour around the pool edge. Be very careful with half used bags of shock. Roll the bag down tightly and clip with a ‘Chip Clip’ or large paperclip, to keep granules from spilling, and to lock out moisture, insects and dirt, which could cause a reaction, possibly igniting or creating noxious gases.

    • Hi Michelle, a normal shock is 1 lb per 10,000 gallons, but in the presence of algae or colored or cloudy water, a 3x or 4x shock dosage is not unusual. If your water is clear, I would use 1/4 cup, or 4 oz of shock (1/4 bag). If there is algae or cloudy water, use 1/2 cup or 1/2 bag, or more if needed. Pre-dissolve by pouring into a 5 gal bucket of water, and stirring to dissolve, then pour around the pool edge. Be very careful with half used bags of shock. Roll the bag down tightly and clip with a ‘Chip Clip’ or large paperclip, to keep granules from spilling, and to lock out moisture, insects and dirt, which could cause a reaction, possibly igniting or creating noxious gases.

  10. I have an 800 gallon pool…I added 2 chlorine tablets to the filter a few days again & another 2 today. The water is slightly cloudy, what do I need to do? If shock is to be used, how much & any other chemicals?

    • Hi Danielle, for clear water, you need good filtration, in addition to good water chemisty, meaning a good pH level 7.2-7.6, and a constant chlorine residual. For shocking such a small pool, bleach is safer to use than the granular shock normally used. Just a cup or two of regular unscented bleach, every week or two, or as needed.

  11. We have a 16,000 gal. Fiberglass inground pool. During the Winter, very high winds blew snow that picked up a tremendous amount of dirt onto our pool cover. We opened the pool yesterday, 6/8 and the water was literally brown. Put 3 bags of In The Swim pool shock in it, today is is green and we can see the cleaner if it is not on the bottom. My question is how much shock can we use and what else can we do. The cleaner bag doesn’t seem to have much yuck in it now & we have hosed the cartridge filters as good as we can twice. Any help would be appreciated. Thanks

    • Hi Beth, if the pool was still green after 3 bags of shock, that just means that you need more shock! add until the water turns a blue/grey color, with no hint of green and – the chlorine level is still high 12 hours later. First check alkalinity and pH levels and adjust pH down to around 7.2-ish. For killing algae, you need 30 ppm of sustained free chlorine, plus good filtration and circulation. Depending on water chemistry, and and the amount of algae, it can take a lot of shock sometimes. Using 4-5 lbs of shock per 10000 gals is not uncommon in my experience, and I’ve used much more at times. An alternative could be to drain and refill 1/3 to 1/2 of the pool water, which will reduce the chlorine demand by the same amount.