Trichlor vs Dichlor

What is the difference between trichlor and dichlor?

Trichlor and dichlor are both stabilized chlorines from the same chemical family, and the difference is the form they come in and what that form is good for. Trichlor is a hard tablet at about 90% available chlorine that dissolves slowly, so it feeds chlorine into the water over days from a floater or feeder. Dichlor is a granular powder at about 56 to 62% available chlorine that dissolves in minutes, so it is what you reach for to shock or to dose a spa. Both carry their own cyanuric acid, the stabilizer that shields chlorine from sunlight, so unlike liquid chlorine or cal hypo, neither one leaves your stabilizer alone.

The pH difference is the other thing that separates them day to day. Trichlor is strongly acidic, around 2.8 to 3, so a season of feeding tablets slowly pulls your pH and total alkalinity down, and you add base to bring them back. Dichlor is nearly neutral, around 6.5 to 7, so it barely moves your balance when you dose with it. That is why trichlor belongs in a feeder doing slow, steady work and dichlor belongs in a scoop doing fast, occasional work.

Because both add stabilizer, the choice between them is really a choice about how you want to deliver chlorine, not whether you want stabilizer. For hands-off daily chlorination, trichlor tablets are the standard, and you can compare the actual products in our best chlorine tablets guide. For a fuller picture of what belongs in your pool and in what order, see pool chemicals: what you actually need, and watch your stabilizer with the cyanuric acid calculator.

What has more CYA, dichlor or trichlor?

Per unit of chlorine, dichlor adds more cyanuric acid: about 0.9 ppm of CYA for every 1 ppm of free chlorine it delivers, versus about 0.6 ppm for trichlor. So dose for dose, matched by the chlorine added, dichlor is the bigger stabilizer source. That surprises people, because trichlor has the reputation for driving CYA up.

The reputation is not wrong, though, because in practice trichlor usually pushes a pool's cyanuric acid higher over a full season. The reason is how it gets used: trichlor is the tablet running in your feeder every day, quietly meeting almost all of your pool's chlorine demand, so far more total chlorine passes through it than through the occasional scoop of dichlor. More total chlorine delivered means more total stabilizer left behind, even though each unit of trichlor adds less. Dichlor's stabilizer stacks up only if you make it your everyday chlorine instead of an occasional one.

Either way, the stabilizer never leaves on its own. Nothing in normal pool chemistry consumes cyanuric acid, so it only goes up until you dilute it. Keep an eye on the number with the cyanuric acid calculator, and if it has already climbed out of range, our how to lower cyanuric acid guide walks through the only real fix.

Is trichlor or dichlor bad for your pool?

Neither one is bad for the job it is designed for; the trouble starts when you push either past its role. Trichlor is excellent for daily chlorination and dichlor is excellent for shocking and spas, and used that way they are cheap, convenient, and reliable. The complaints come from using a stabilized chlorine as if it added no stabilizer.

Trichlor's two real downsides are that it steadily raises cyanuric acid and that it is acidic. Let CYA climb too high and your chlorine starts to underperform: the test still shows chlorine present, but it works sluggishly and algae can take hold anyway, a situation people call chlorine lock or overstabilization. The acidity is easier to manage, you just add base to hold pH and alkalinity in range, and you can size that correction with the pool pH calculator. Both problems are why trichlor is the wrong everyday choice for a pool whose CYA is already high.

Dichlor's downside is the same stabilizer creep if you overuse it, plus a higher price per pound of chlorine. Its low pH and clean dissolving make it genuinely pleasant to use, but that convenience is not a reason to run it as your constant chlorine source. The honest read on both: they are good tools used correctly, and the moment your stabilizer is in range and climbing, switch daily chlorination to an unstabilized chlorine and keep these two for their specialty jobs. See the target numbers in ideal pool chemistry levels.

Trichlor or dichlor: which should you use?

Use trichlor for everyday chlorination of an outdoor pool. Its slow-dissolving tablets give you hands-off, steady chlorine from a floater or feeder for days at a time, it is the cheapest common chlorine per pound, and its built-in stabilizer keeps the sun from burning your chlorine off. Accept its two quirks, that it slowly lowers pH and alkalinity and slowly raises CYA, keep both in check, and it is the most convenient way to keep a normal in-ground or above-ground pool chlorinated.

Use dichlor when you want a fast granular chlorine that will not upset your balance: shocking after heavy use, dosing a hot tub or spa, chlorinating a brand-new pool that needs its first stabilizer, or a quick spot dose in cold water where tablets barely dissolve. In those jobs its near-neutral pH and clean, fast dissolving are exactly right, and you drain or dilute often enough (or dose rarely enough) that the added stabilizer never becomes a problem. Compare it against the stronger unstabilized shock in our cal hypo vs dichlor comparison.

The rule that overrides both: once your cyanuric acid is in range and still climbing, stop using stabilized chlorine for daily work and move to an unstabilized one, either liquid chlorine or cal hypo. And one safety rule for both trichlor and dichlor: never mix them, never load them into the same feeder or floater, never put either in the skimmer, and never pour one on top of the other, because combining chlorine products can react violently or release toxic gas. Size any dose to your real water volume with the pool volume calculator and the pool chlorine calculator.

Frequently asked questions