What is the difference between cal hypo and dichlor?
The difference is what each one leaves behind. Both are granular chlorine and both will sanitize and shock a pool, but cal hypo adds calcium to your water and dichlor adds cyanuric acid, the stabilizer that protects chlorine from sunlight. That single distinction drives almost every choice below, because calcium and cyanuric acid behave very differently once they build up.
Cal hypo is calcium hypochlorite, an unstabilized chlorine that runs about 65 to 73% available chlorine, the strongest common granular shock. It is highly alkaline, around 11.7, so a heavy dose pushes pool pH up and often needs acid afterward to bring it back. It is also the cheapest chlorine per pound, which is why it is the default pool shock. The catch is that it must be pre-dissolved in a bucket of water before it goes in, because dry granules can settle on a vinyl liner and bleach it, and it slowly raises calcium hardness over a season.
Dichlor is a stabilized chlorine at about 56 to 62% available chlorine, weaker than cal hypo but still strong. It is nearly pH-neutral, around 6.5 to 7, so it barely moves your balance, and it is fully soluble, so it dissolves fast and clean with no calcium residue. That makes it convenient, but every dose carries stabilizer into the water. See how tablets do the same thing in our liquid chlorine vs tablets comparison, and size any dose with the pool shock calculator.
Does dichlor raise cyanuric acid, and does cal hypo add calcium?
Yes to both, and this is the heart of the decision. Every dose of dichlor leaves behind almost as much cyanuric acid as the chlorine it adds, so repeated use quietly climbs your stabilizer. Cal hypo adds no stabilizer at all; instead it slowly raises calcium hardness. Neither buildup shows up overnight, but both matter over a season, and they push you toward opposite products.
Runaway cyanuric acid is the bigger trap. Once CYA drifts too high, it over-protects the chlorine and your sanitizer stops working even though the test shows chlorine present, a state people call chlorine lock. The only real fix is draining and refilling to dilute it. That is why dichlor is a poor everyday choice for a pool that already has stabilizer in range: you would be adding a problem you then have to drain away. Watch the level with the cyanuric acid calculator.
Cal hypo's calcium is the gentler buildup for most people, but it is not free. In hard-water areas or a pool already high in calcium, repeated cal hypo shocking can push hardness toward scaling and cloudy water, especially paired with its high pH. If you are on soft water or your calcium runs low, that same calcium is actually a small bonus. Either way, keep an eye on the balance and let the pool shock calculator size the dose so you add only what the water needs.
Which is better for shocking a pool, cal hypo or dichlor?
For shocking a normal pool, cal hypo wins. It carries the most chlorine per pound, costs the least, and adds no stabilizer, so you can shock all season without your cyanuric acid creeping upward. When the job is to blast a cloudy or green pool back to clear, or to hit breakpoint after heavy use, cal hypo is the strong, cheap, sensible choice. Just pre-dissolve it, broadcast it in the evening, and run the pump. Walk through the full routine in our how to shock a pool guide.
Dichlor is the weaker shock and the wrong tool for constant superchlorination, precisely because it stabilizes. Shock a pool with dichlor a few times a summer and you can drive CYA past the point where chlorine works, then face a drain-and-refill to fix it. The exception is a brand-new pool or a pool with zero stabilizer: there, an early dichlor dose does double duty by shocking and building the CYA you need anyway. After that, switch to an unstabilized shock.
Watch the pH difference when you shock. Cal hypo is highly alkaline, so a big shock dose spikes pH and you often chase it with acid to get back in range; keep the pool pH calculator handy after a heavy cal hypo shock. Dichlor barely moves pH, which is genuinely nice, but that convenience is not worth the stabilizer it stacks up in a pool that already has enough. For specific products by job, see our best pool shock picks.
Cal hypo or dichlor: which should you use?
Use cal hypo for a normal pool. It is the standard shock for a reason: strongest chlorine per pound, lowest cost, and no stabilizer to build up, which is exactly what you want when your CYA is already in range and you shock regularly. Accept its two quirks, the high pH and the need to pre-dissolve, and it is the most cost-effective way to keep an in-ground or above-ground pool clear.
Use dichlor when you actually want what it adds. A hot tub or spa is its home turf, where its near-neutral pH and clean dissolving keep small, frequent doses simple and where you drain often enough that stabilizer buildup never becomes a problem. It also fits a new pool that needs its first stabilizer, or a small above-ground pool where convenience beats price. Outside those cases, dichlor's stabilizer is a liability, not a feature.
One safety rule for both: never mix them dry, never load them into the same feeder or floater, and never pour one on top of the other. Combining chlorine products can react violently. Add them to the pool separately, pre-dissolved, at different times. Size whichever you pick to your water volume with the pool volume calculator and the pool shock calculator so you dose right the first time.
Cal Hypo wins on
- +Strongest common shock at about 65 to 73% available chlorine, and the cheapest per pound.
- +Adds no stabilizer, so you can shock all season without cyanuric acid climbing toward chlorine lock.
- +The standard, widely available shock for a normal in-ground or above-ground pool.
Dichlor wins on
- +Nearly pH-neutral and fully soluble, so it dissolves clean with no residue and no liner bleaching.
- +Carries its own stabilizer, ideal for a new pool with low CYA or small, frequent doses.
- +The right granular chlorine for hot tubs and spas, where its clean, gentle profile shines.
The verdict
For shocking a typical pool, reach for cal hypo. It packs the most chlorine per pound, costs the least, and adds no cyanuric acid, so you can shock all season without your stabilizer creeping toward chlorine lock. Its only real quirks are a sharp pH rise, which you correct with acid, and the need to pre-dissolve it before it goes in. Choose dichlor when you specifically want what it adds: a hot tub or spa, a brand-new pool that has no stabilizer yet, or a small pool where near-neutral pH and clean dissolving matter more than price. The trap with dichlor is using it repeatedly in a pool that already has enough CYA, which quietly climbs your stabilizer until chlorine stops working and only a drain fixes it. Track your levels with the cyanuric acid calculator, size any dose with the pool shock calculator, and compare specific products in our best pool shock guide.
Related: Best pool shock, Pool shock calculator, How to shock a pool.
Frequently asked questions
Should I use dichlor or trichlor?
It depends on how you want to add it. Both are stabilized chlorines that add cyanuric acid, but dichlor is a fast-dissolving granular you broadcast for shocking or quick dosing, and it is nearly pH-neutral. Trichlor is a slow-dissolving 3-inch tablet you set in a feeder or floater for hands-off daily chlorination, and it is strongly acidic, so it lowers pH over time. Use dichlor for granular shock and spa dosing, and trichlor for steady, low-effort chlorination. Both build CYA, so watch your stabilizer level with either one.
Can I use dichlor and cal hypo together?
Not in the same container, ever. Mixing chlorine products dry, or loading them into the same feeder or floater, can trigger a violent reaction or fire, because cal hypo is an oxidizer and dichlor is organic chlorine. You can use both in the same pool if you add them separately, pre-dissolved, at different times: for example dichlor to build early stabilizer and cal hypo to shock later. Just never combine them in a bucket, and rinse any scoop between products.
Can you put cal hypo in the skimmer?
No. Never put cal hypo, or any granular shock, in the skimmer. Undissolved granules can settle and damage the pump and heater, the high pH and calcium concentrate right at your equipment, and if any trichlor residue is in the lines the two can react and produce toxic chlorine gas. Instead, pre-dissolve cal hypo in a bucket of water and broadcast it around the pool with the pump running, or pour the dissolved solution slowly around the perimeter.
Should I use cal hypo or liquid chlorine?
Both are unstabilized, so neither builds cyanuric acid, and that is their shared advantage. Liquid chlorine (sodium hypochlorite) is easier to use: no pre-dissolving, no calcium added, and it pours straight in, though it is bulky to store and loses strength on the shelf. Cal hypo is more concentrated and cheaper to buy and store, but it adds calcium and must be dissolved first. Use liquid for easy regular chlorination and cal hypo for a cheap, strong shock when your calcium hardness is not already high.
Does cal hypo raise pH?
Yes, noticeably. Cal hypo is highly alkaline, around 11.7, so a shock dose pushes pool pH up, and after a heavy shock you often add acid to bring it back into the 7.2 to 7.8 range. Dichlor, by contrast, is nearly neutral at around 6.5 to 7 and barely moves pH. If you shock with cal hypo, retest pH once the chlorine settles and correct it with the help of the pool pH calculator.