Quick Reference
- Chemical name: Sodium dichloro-s-triazinetrione (sodium dichloroisocyanurate)
- Available chlorine: typically 55-56% (dihydrate), 60-62% (anhydrous)
- pH: mildly acidic to near-neutral, roughly 6-7 as a 1% solution. Much less acidic than trichlor
- Adds CYA: yes, about 0.9 ppm CYA per 1 ppm free chlorine
- Form: granular, dissolves quickly
- Best for: spa startup, new pools at zero CYA, vinyl and fiberglass surfaces
- Worst for: day-to-day pool chlorination and shocking
Table of Contents
What Dichlor Is
Dichlor is granular stabilized chlorine. Like trichlor, the chlorine and the cyanuric acid come packaged in the same molecule, so every dose adds both. Unlike trichlor, dichlor is close to pH-neutral and dissolves in under a minute instead of over days.
That combination (granular, fast, not very acidic) makes it the most convenient chlorine on the shelf. It is also the reason people get into trouble with it.
The CYA Problem
Every 1 ppm of free chlorine you add with dichlor also adds roughly 0.9 ppm of cyanuric acid. Cyanuric acid is stubborn. In a normal pool it leaves mainly through water replacement: splashout, backwashing, or a partial drain.
Run a pool on dichlor as its main sanitizer all season and CYA can climb well past 100 ppm. At that point your chlorine is over-stabilized. You need a much higher free chlorine level to get the same sanitizing punch, algae gets a foothold at numbers that used to be fine, and the practical fix is draining part of the water and refilling.
Worth knowing: per unit of chlorine, dichlor raises CYA faster than trichlor does. Trichlor runs about 0.6 ppm CYA per 1 ppm FC; dichlor is about 0.9. Both numbers are rules of thumb and vary a little by product.
When Dichlor Is the Right Call
- Starting up a spa. The "dichlor then bleach" method uses dichlor on a fresh fill until CYA tests around 20-30 ppm, then switches to liquid chlorine so CYA stops climbing for the rest of that water's life.
- A brand-new pool at zero CYA, where you want chlorine and stabilizer at the same time.
- Vinyl liners and fiberglass. Dichlor's near-neutral pH is easier on those surfaces than acidic trichlor sitting in a floater.
- When you need chlorine right now and cal hypo would push calcium hardness higher than you want.
Pro tip: buy plain dichlor. A lot of spa products labeled "shock" are dichlor blended with clarifiers, brighteners, or fragrance. You are paying for additives you did not ask for and cannot dose independently.
When to Reach for Something Else
- Routine chlorination of an established outdoor pool. Use liquid chlorine. It adds no CYA.
- Shocking. See superchlorination. Shocking with dichlor means dumping a large slug of CYA in one dose, which is exactly backwards: you are raising the stabilizer at the same moment you need chlorine to be aggressive.
- Any pool already sitting at 50 ppm CYA or higher, unless you specifically want to raise CYA further.
Dichlor vs. Other Chlorines
| Product | Available Chlorine | Effect on pH | Adds CYA | Typical Use |
|---|---|---|---|---|
| Dichlor | 55-62% | Slight drop | Yes, ~0.9 per 1 FC | Spa startup, new fills |
| Trichlor | 90% | Strong drop | Yes, ~0.6 per 1 FC | Tablets in a feeder |
| Liquid chlorine | 10-12.5% | Slight rise | No | Daily dosing, shocking |
| Cal hypo | 65-73% | Rise | No (adds calcium) | Shocking |
Spa and Hot Tub Notes
Spa volumes are small, so a heaping scoop is a big dose. Measure rather than eyeball, and test after adding rather than assuming.
If your spa runs on bromine, dichlor still has a role: the chlorine in it oxidizes bromide in the water back into active bromine sanitizer. But understand what you are doing. You are feeding a bromine system, not running a chlorine one, and you are still adding CYA along the way.
Handling and Safety
Follow the product label first. Labels differ on whether to broadcast granules directly or pre-dissolve, and the label is the legally binding instruction. Everything below is general practice, not a substitute for it.
- Never mix dichlor with calcium hypochlorite, and never store them where a spill could bring them together. Combining chlorinated isocyanurates with cal hypo can cause a violent reaction, fire, and release of hazardous chlorine-containing gas. Use separate, dedicated scoops.
- Add with the pump running so it disperses.
- Do not let granules sit on a vinyl liner, steps, or a fiberglass surface. Brush any that settle.
- Keep it dry and sealed. Moisture degrades it and can start reactions in the container.
- Gloves and eye protection. Store locked, away from kids, pets, acids, and anything combustible.
For a deeper community-tested treatment of stabilized chlorine and CYA management, Trouble Free Pool is the best free resource out there.