Cyanuric acid is the reading most pool owners never look at, and it is the one that quietly decides whether their chlorine works at all.
What it actually does
Sunlight destroys free chlorine. In an unstabilised outdoor pool, a sunny afternoon can remove most of it. Cyanuric acid — stabiliser, or conditioner — binds loosely to chlorine and shields it from UV, so the chlorine survives the day instead of burning off by lunchtime.
The catch is that bound chlorine is not working chlorine. Stabiliser protects your chlorine by holding it in reserve, which means the more stabiliser you have, the more total chlorine you need in the water to get the same sanitising effect.
The relationship nobody tells you about
Free chlorine cannot be judged on its own. A reading of 2 ppm is comfortable at 30 ppm CYA and close to useless at 80. The widely used working rule is that free chlorine should be kept at roughly 7.5% of the CYA reading as a minimum, and that is the number to hold, not a fixed 1–3 ppm.
| CYA (ppm) | Minimum free chlorine | Shock level |
|---|---|---|
| 30 | 2.3 ppm | 12 ppm |
| 40 | 3.0 ppm | 16 ppm |
| 50 | 3.8 ppm | 20 ppm |
| 60 | 4.5 ppm | 24 ppm |
| 80 | 6.0 ppm | 32 ppm |
The shock column is the same relationship at the other end: an effective shock is about 40% of the CYA reading. This is why shocking a high-stabiliser pool with a standard dose does nothing — the dose was never large enough to matter.
How it creeps up without you adding any
Most people who end up with 100 ppm of stabiliser never bought a bag of it. Trichlor tablets and dichlor granules both contain cyanuric acid, so every tablet adds a little. It accumulates all season, and unlike chlorine it does not burn off, evaporate or break down.
There is no chemical that removes cyanuric acid. The only practical way down is to drain part of the pool and refill. That is the whole reason to watch the number before it becomes a problem.
What to aim for
- Outdoor chlorine pool: 30 to 50 ppm.
- Saltwater pool: often 60 to 80 ppm, because the cell produces chlorine continuously and the extra protection helps it keep up.
- Indoor pool: none. Without sunlight there is nothing to protect against, and stabiliser only slows the chlorine down.
Calculators
The pattern that gives it away
If chlorine reads fine in the morning and nearly nothing by evening, you probably have too little stabiliser. If chlorine reads fine and the water still goes cloudy or green, you probably have too much. Both look like a chlorine problem and neither is one.
Water Health Check
Enter your test results and see what is in range, what is not, and what to do first.
CalculateHow often should I test cyanuric acid?
Monthly is enough — it moves slowly. Test it after any significant water replacement, and at the start of each season.
My CYA is 100 ppm. What now?
Partial drain and refill, and there is no way around it. Replacing a third of the water cuts the reading by roughly a third. In the meantime hold chlorine far higher than you normally would.
Can I just stop using tablets?
That stops it climbing, which is the first step. Liquid chlorine and cal-hypo add none. But the stabiliser already in the water stays there until it is diluted out.
Is high CYA dangerous?
Not to swimmers directly. The risk is indirect: it makes chlorine ineffective at the levels most people hold, which is how a pool ends up under-sanitised while its chlorine reading looks fine.