What Is Dichlor? Stabilized Chlorine Shock Explained
Quick answer: Dichlor (sodium dichloroisocyanurate) is a stabilized chlorine shock with approximately 56% available chlorine. Every dose adds both Free Chlorine and Cyanuric Acid: roughly 0.9 ppm CYA per 1 ppm FC added. It dissolves fast, is pH-neutral, and is the default shock for residential hot tubs. For outdoor pools past initial CYA setup, switch to liquid chlorine to prevent CYA from climbing out of range.
Understanding which shock to reach for is a core part of pool chemistry. Dichlor sits in an interesting position: it’s the most convenient product for hot tub owners and new pool startups. But it becomes a liability in established outdoor pools where CYA is already at target. Choosing it for the wrong application leads to low chlorine in pool problems that are hard to diagnose without knowing CYA is the culprit.
Dichlor vs. other shocks
The shock category includes four distinct products with different chemistry profiles. The comparison matters because the wrong choice in the wrong pool creates problems that take weeks to correct.
Common mistake.
| Shock Type | Available Chlorine | pH | Adds to Water | Best Use |
|---|---|---|---|---|
| Dichlor | ~56% | Neutral (~6.9) | CYA (+0.9 ppm/ppm FC) | Hot tubs, vinyl pool startup |
| Cal-Hypo | 65-73% | Very basic (11-12) | Calcium hardness | Outdoor pools, algae treatment |
| Tri-chlor tablets | ~90% | Very acidic (2.8) | CYA (+0.6 ppm/ppm FC) | Maintenance dosing via floater |
| Liquid chlorine | 10-12.5% | Basic (12-13) | Nothing extra | Best all-around for outdoor pools |
Per EPA chlorine disinfection guidance{:target=“_blank”}, all forms of chlorine provide equivalent microbial kill at the same FC level. The choice between them comes down to what byproducts you can tolerate in your specific pool chemistry context.
Pretty simple.
Dichlor’s pH-neutral profile makes it genuinely useful. Cal-hypo at pH 11-12 requires acid adjustment after every shock application. Dichlor at neutral pH means a balanced pool stays balanced after treatment, without the correction step. That convenience is why pool stores often recommend it broadly, even in situations where the CYA accumulation makes it problematic.
When to Use Dichlor
Dichlor is the right product in specific situations. Outside those situations, we recommend liquid chlorine.
Residential hot tubs. Dichlor is the default shock for most residential hot tubs. Hot tub water isn’t under direct sunlight for extended periods, so CYA buildup from dichlor is slower. The pH-neutral profile means less adjustment work in the small water volume of a spa. The granular form dissolves fast and distributes well in the circulating water. For hot tub chemicals management, dichlor is the most practical choice for occasional shocking.
New pool startup. When filling a new pool or after a significant drain-and-refill, both FC and CYA start at zero. Dichlor covers both simultaneously. This is a legitimate use case because you need stabilizer regardless. The combined dose is convenient. Stop using dichlor once CYA reaches 30-50 ppm, then switch to liquid chlorine for ongoing maintenance.
Short travel periods. Throwing a measured dose of dichlor into a residential hot tub or small pool before a week away provides sustained chlorine. The CYA boost from one or two applications isn’t significant.
What to avoid: Using dichlor as the primary chlorine source for an established outdoor pool. Once your CYA is already in the 30-70 ppm target range, every dichlor dose pushes CYA higher without you getting anything useful from the addition. The CDC pool disinfection standards{:target=“_blank”} are based on maintaining adequate FC, which becomes harder when CYA climbs past 70-80 ppm.
Dichlor dosage
Dichlor (sodium dichloroisocyanurate) at around 56% available chlorine, serves as a stabilized shock treatment; adding each 1 ppm of free chlorine introduces roughly 0.9 ppm of cyanuric acid.
A 5-lb bag of 56% dichlor granular typically runs $12-$20 at pool supply retailers. For practical dosing, the InTheSwim charts give these numbers for a 56% dichlor product:
- 10,000-gallon pool, preventative shock to 10 ppm FC: 1.4 lb dichlor (also adds ~6 ppm CYA)
- 10,000-gallon pool, algae treatment to 30 ppm FC: 4.2 lb dichlor (also adds ~18 ppm CYA)
- 5,000-gallon pool, algae treatment to 30 ppm FC: 2.1 lb dichlor
For hot tubs, product labels specify dose by volume. Most residential spas (350-500 gallons) need 1-2 teaspoons of granular dichlor for maintenance shocking.
Note that using dichlor for a full algae-treatment dose in a 10,000-gallon pool adds 18 ppm CYA in a single application. If CYA was already at 50 ppm, it’s now at 68. Do this twice in a season and CYA exceeds 80 ppm. For outdoor pools with algae problems, pool stabilizer (cyanuric acid protector) levels should be checked before every shock application.
CYA buildup warning
CYA can’t be removed chemically. The only way to lower it’s to drain water and replace it with fresh water. That constraint makes CYA accumulation a genuine problem for dichlor users who don’t track it.
Common mistake.
Over a swim season, weekly dichlor shocking pushes CYA steadily upward. At 0.9 ppm CYA per ppm FC added, a weekly 10-ppm shock in a 10,000-gallon pool adds about 6 ppm CYA per week. Over a 20-week season, that’s 120 ppm added to whatever baseline you started with. Even starting at zero, you would end the season at CYA 120 ppm.
The TroubleFreePool FC/CYA chart shows what happens at high CYA: the minimum FC needed to prevent algae rises proportionally. At CYA 80 ppm, the TFP minimum FC is 6 ppm. At CYA 100 ppm, it reaches 7-8 ppm. The pool becomes very difficult to maintain, and the chlorine smell that many people associate with “too much chlorine” (actually combined chlorine, not free chlorine) gets worse.
Dichlor is the best general-purpose shock for residential hot tubs. It shouldn’t be the primary chlorine source for outdoor pools past initial CYA buildup because CYA climb becomes unmanageable over a full season. For technique on actually applying shock, see our how to shock a pool guide.
Storage and safety
Dichlor is sold as a granular powder or compressed granules. It’s stable when kept dry and sealed, but storage practices matter.
Keep dry. Moisture starts the dissolving reaction inside the container. Wet dichlor releases chlorine gas and can clump into a solid mass. Store in a cool, dry location away from direct sun.
Never mix with cal-hypo in storage. Dichlor and cal-hypo are chemically incompatible. Mixing them directly or storing them in contact can cause combustion or release toxic chlorine gas. Store them in separate, clearly labeled containers.
Don’t mix with tri-chlor. The same incompatibility applies. Mixing oxidizers from different chemical families is a fire and toxic gas risk.
pH neutral in water. Once dissolved in the pool, dichlor’s neutral pH isn’t a handling hazard. Unlike cal-hypo (which can ignite if it contacts pool water with high organic load), dichlor is the gentler product to handle and add.
FAQ
Can I use dichlor in a saltwater pool?
We don’t recommend it. Saltwater pools already have CYA from the stabilized environment. The salt chlorine generator provides FC continuously. Adding dichlor raises CYA further without providing any benefit the SWG can’t handle. If you need to shock a saltwater pool, use liquid chlorine (sodium hypochlorite) for the cleanest addition.
Does dichlor expire?
Properly stored dichlor, ideally in cool, dry conditions and sealed tightly, retains its effectiveness for 3 to 5 years. Clumps or yellowing indicate degradation; check by opening the container, strong chlorine smell suggests active ingredient remains. Faint odor or none? Test a small dose: if it fails to dissolve fully or raise Free Chlorine (FC) levels, opt for fresh product.
Which shock is best for hot tubs?
For residential hot tubs, dichlor is our recommendation for ongoing shocking. It’s pH-neutral (important in small water volumes), dissolves fast, provides immediate FC, and adds a modest CYA boost that helps protect the chlorine from UV degradation in uncovered spas. MPS (non-chlorine shock) is an alternative for weekly oxidation between dichlor doses, but MPS doesn’t kill algae or provide the same sanitizing power. For hot tub sanitizer options beyond chlorine, bromine is the primary alternative.