Best Pool Shock
The best pool shock for most people is unstabilized calcium hypochlorite (cal-hypo): it is cheap, hits hard, and does not add cyanuric acid that builds up over a season. The catch is that shock is not one product. There are three chemistries, and the right one depends on whether you are killing algae, doing routine maintenance, or trying to swim again within the hour. Here are the picks by job, and how to choose.
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In The Swim Calcium Hypochlorite Pool Shock
Who it is for: Most outdoor chlorine pools that want a strong, no-frills shock without adding stabilizer.
- +High available chlorine per bag, so it raises free chlorine fast.
- +Unstabilized, so it does not push up your cyanuric acid the way di-chlor does.
- +Sold in cases of pre-measured 1 lb bags, which makes dosing by pool volume simple.
Watch out: It adds a little calcium each time, so watch hardness in already-hard water. Predissolve it in a bucket and add at dusk, since direct sun burns off chlorine fast.
DryTec Calcium Hypochlorite Shock Treatment
Who it is for: Anyone fighting algae or clearing a cloudy pool, where you need a high chlorine level held for days.
- +Concentrated cal-hypo dissolves cleanly and reaches algae-killing chlorine levels.
- +Fast-acting, which matters when you are running a green pool back to clear.
- +Granular form lets you broadcast over a large surface or target a stubborn corner.
Watch out: Clearing algae takes repeated doses, not one. Hold the level and retest before adding more, and brush daily. See the chlorine math on the pool shock calculator.
Sodium Di-Chlor Granular Shock
Who it is for: Outdoor pools that shock weekly and want the stabilizer to protect chlorine from the sun.
- +Di-chlor is pH-neutral and dissolves fast, so it is gentle on water balance.
- +The built-in stabilizer (cyanuric acid) slows chlorine loss in direct sunlight.
- +Convenient for regular top-up shocking through the swim season.
Watch out: Every dose adds cyanuric acid, which builds up and eventually weakens your chlorine. Track it with the cyanuric acid calculator and switch to cal-hypo once CYA climbs.
In The Swim Chlorine-Free Pool Shock Oxidizer
Who it is for: People who want to oxidize away sweat, oils, and odors and swim again within about 15 minutes.
- +Potassium monopersulfate (MPS) clears organic waste without raising chlorine.
- +No wait for chlorine to drop, so you can shock in the morning and swim by midday.
- +Adds no cyanuric acid and no calcium, so it does not skew your balance.
Watch out: It oxidizes but does not sanitize, so it will not kill algae or replace your regular chlorine. Use it for maintenance and clarity, not to rescue a green pool.
What actually matters when buying
Know the three chemistries. Cal-hypo (calcium hypochlorite) is the strong, cheap, unstabilized workhorse. Di-chlor is stabilized and pH-neutral, easy for routine shocking but it raises CYA. Non-chlorine shock (MPS) oxidizes waste for fast reswim but does not sanitize or kill algae. Match the chemistry to the job.
Read the percent available chlorine. Not all bags are equal. Cal-hypo runs anywhere from roughly 47 to 73 percent available chlorine, so a cheaper bag may simply contain less active product. Compare the percentage, not just the price per pound, and dose by your actual water volume.
Stabilized shock raises your CYA. Di-chlor and trichlor add cyanuric acid every time. Over a hot season of weekly shocking that can climb high enough to lock up your chlorine, and the only fix is draining and refilling. If you already shock with stabilized chlorine, lean on unstabilized cal-hypo or non-chlorine shock instead.
Cal-hypo adds calcium. Calcium hypochlorite leaves calcium behind. In soft water that is harmless, but in hard water it slowly raises calcium hardness and can contribute to scale. Check your hardness with the calcium hardness calculator if you shock heavily with cal-hypo.
Dose by volume, then test. Shock is sized to gallons, so get your pool volume right first and never eyeball it. Work out the dose on the pool volume calculator and the shock calculator, add it at dusk, run the pump, and retest before adding more.
How we picked
This is a research-based guide comparing shock chemistries (cal-hypo, di-chlor, and non-chlorine MPS), available-chlorine strength, effect on cyanuric acid and calcium, and a broad set of owner reviews across the established shock brands. We do not bench-test products, and we do not take payment for placement. Chemistry claims reflect well-documented differences between these sanitizer types. Always dose to your tested water and follow the label.
Keep your water right, too
Gear handles the cleaning; chemistry is the other half. Useful next: how to shock a pool, pool shock calculator, chlorine calculator, all calculators.
Frequently asked questions
Which pool shock is the best?
For most outdoor pools, unstabilized calcium hypochlorite (cal-hypo) is the best all-round shock: it is strong, inexpensive, and does not build up cyanuric acid. Use a high-strength cal-hypo for green or cloudy water, a stabilized di-chlor for easy weekly maintenance, and a non-chlorine MPS oxidizer when you want to swim again within minutes.
Are chlorine and pool shock the same thing?
Shock is chlorine in most cases, just a large, fast dose meant to spike your free chlorine and burn off contaminants. The exception is non-chlorine shock (potassium monopersulfate), which oxidizes waste without adding chlorine at all. Regular chlorine tablets sanitize day to day; shock is the periodic reset.
When should you put shock in your pool?
Shock after heavy use, a rainstorm, a pool party, or any time your water turns cloudy or green, and on a routine weekly basis in peak season. Always add it at dusk or after dark, because direct sunlight burns off unstabilized chlorine before it can do its work. Run the pump for several hours afterward.
Can you add shock and chlorine at the same time?
Do not mix different chemicals together in the same bucket or floater, since concentrated chlorine products can react violently. You can run tablets in your feeder and still shock the pool, but add the shock separately, predissolved, and give the water time to circulate. Never combine cal-hypo and trichlor directly.
Which pool shock is better, HTH or Clorox?
Both HTH Super Shock and Clorox Pool and Spa shock are calcium hypochlorite products that work well, so the practical difference is the percent available chlorine on the bag and the price per pound. Compare those two numbers rather than the brand. A higher-percentage cal-hypo gives you more active chlorine per dollar.