What shocking actually does
Every day your chlorine reacts with sweat, sunscreen, urine, leaves, and other organic matter, and some of it gets tied up as combined chlorine (chloramines). Combined chlorine is the stuff that makes eyes sting and gives a pool that strong chlorine smell. Shocking pushes free chlorine high enough to break those compounds apart and clear them out, which is why people call it superchlorination.
So shock is not a different chemical from your normal sanitizer, it is the same chlorine at a much higher dose. The goal is to hit a target level and let it work, not to dump a random bag in and hope. How much you need depends on your pool volume and your cyanuric acid level, which is why guessing usually under-doses.
When to shock
Shock after anything that loads the water: a big pool party, a heavy rain, a heat wave, or when the water turns cloudy or green. A good routine trigger is combined chlorine reading above 0.5 ppm, which means chloramines are building up. Many owners also shock every week or two in peak season as maintenance, though a pool with steady, correct chlorine often does not need a fixed schedule.
Always shock in the evening or after sunset. Sunlight burns off free chlorine fast, so shocking at noon wastes a chunk of the dose before it can work. Adding it at dusk and running the pump overnight gives the chlorine all night to do its job.
Pick the right shock for the job
For routine shocking and especially for clearing algae, liquid chlorine (sodium hypochlorite) or plain unscented bleach is the cleanest choice because it adds nothing but chlorine. Cal-hypo (calcium hypochlorite) is a strong granular shock but adds calcium hardness, so it is a poor pick if your calcium is already high. Dichlor dissolves easily but adds cyanuric acid every time, which slowly drives your CYA up and makes future chlorine weaker.
If you have hard water or high CYA already, lean liquid. If you have soft water and want a granular product, cal-hypo is fine. Avoid dichlor as a regular shock unless your CYA is low and you want to nudge it up. You can check where you stand with the cyanuric acid calculator before deciding.
How much shock to add
The right dose depends on two numbers: how many gallons your pool holds and your current cyanuric acid level. If you do not know your volume, work it out first with the pool volume calculator, because an over- or under-estimate throws every dose off. For a routine shock you are aiming to roughly triple your free chlorine; for killing algae you need a much higher target tied to your CYA.
Rather than eyeball it, put your numbers into the pool shock calculator to get the target free chlorine and how much of your chosen product reaches it. Add chemicals in stages, run the pump, and retest before adding more. Never treat one calculated dose as final without testing the water afterward.
Step by step
First, test the water and note your free chlorine, combined chlorine, and pH. Brush the walls and floor if the pool is cloudy or has algae. Make sure the pump is running and will keep running for several hours. If you are using granular cal-hypo, pre-dissolve it in a bucket of pool water and pour the liquid in, since undissolved granules can sit on a vinyl liner and bleach it.
Pour the shock slowly around the perimeter of the deep end with the pump on so it circulates. Let it run overnight. The next day, retest: free chlorine should be elevated and dropping. For a normal shock, wait until free chlorine falls back to your normal range before swimming. For algae, keep re-dosing to the shock level until the water is clear and chlorine holds overnight. When everything is back in range, resume normal dosing with the chlorine calculator.
Frequently asked questions
What's the proper way to shock a pool?
Test the water, balance pH first, and shock in the evening with the pump running. Calculate the dose from your pool volume and cyanuric acid level, pre-dissolve granular shock in a bucket if you use cal-hypo, then pour it slowly around the deep end and let the pump circulate it overnight. Retest the next day and wait for free chlorine to drop to your normal range before swimming.
Is shock the same as chlorine?
Shock is chlorine, just a large dose of it. Most pool shock is sodium hypochlorite (liquid chlorine), calcium hypochlorite (cal-hypo), or dichlor. The word shock describes the action of raising free chlorine far above normal to burn off contaminants and kill algae, not a separate chemical from your regular sanitizer.
Can I pour shock directly into the pool?
Liquid chlorine can be poured straight in, slowly, around the deep end with the pump running. Granular cal-hypo should be pre-dissolved in a bucket of pool water first, because undissolved granules can settle on a vinyl liner and bleach or damage it. Always add shock with the pump on so it circulates instead of pooling in one spot.
Do I run my pump when I shock my pool?
Yes. Run the pump while you add shock and for several hours after, ideally overnight. Circulation spreads the chlorine evenly so it reaches all the contaminants and does not sit in one area. Shocking with the pump off leaves high-chlorine pockets and wastes much of the dose.
How long after shocking can you swim?
Wait until free chlorine drops back to your normal range, usually under about 4 ppm, before swimming. After a standard shock that often takes several hours to a day with the pump running. Test before you get in rather than guessing by time, since the amount of shock and sunlight both change how fast it falls.