Here is the whole idea in one sentence: cyanuric acid protects chlorine from sunlight by holding most of it in reserve, and chlorine held in reserve isn't sanitizing anything. That's the bargain. Everything else on this page — the moving FC target, "chlorine lock," pools that go green at textbook chlorine levels — follows from it.
What stabilizer actually does
First, a naming problem worth clearing up: cyanuric acid, stabilizer, conditioner and CYA are all the same chemical. Bags labeled "pool conditioner" and "chlorine stabilizer" are cyanuric acid; so is the CYA already inside trichlor pucks and dichlor granules. The different names sell the same compound, and buying "conditioner" when your CYA is already high is one of the more expensive ways to make a pool problem worse.
Unprotected chlorine has a brutal day. In full summer sun, roughly half the free chlorine in an unstabilized pool is destroyed by UV in well under an hour. You could dose in the morning and have nothing left by lunch.
Cyanuric acid fixes that by bonding loosely with chlorine to form chlorinated isocyanurates — a shaded reserve tank. UV can't easily break those bonds, so the chlorine survives the day and is released gradually as the unbonded portion gets used up. A stabilized pool holds chlorine several times longer than one without.
The catch is in the word bonded. At any moment, only a small fraction of your free chlorine is the active, germ-killing form (hypochlorous acid). The rest is parked in reserve. Add more CYA and you add more parking spaces — the reserve grows, the active portion shrinks.
An FC test reports all your chlorine — reserve and active together. It has no way to tell you how much is actually working, and that fraction depends entirely on your CYA. This is why two pools can both read "4 ppm free chlorine" while one is spotless and the other is turning green: same reading, different amounts of chlorine actually on duty.
Which is why it's a ratio, not a number
If CYA decides what fraction of your chlorine is active, then holding a fixed FC number makes no sense — you have to scale FC with CYA to keep the active amount roughly constant. That's exactly what the Trouble Free Pool FC/CYA relationship encodes, and it's why this site never quotes a single chlorine target:
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| CYA | Minimum FC | Target FC | Shock level | Minimum as % of CYA | Target as % of CYA |
|---|---|---|---|---|---|
| 20 ppm | 2 | 4 | 10 | 10% | 20% |
| 30 ppm | 2.5 | 4 | 12 | 8% | 13% |
| 40 ppm | 3 | 4.5 | 16 | 8% | 11% |
| 50 ppm | 4 | 6 | 20 | 8% | 12% |
| 60 ppm | 4.5 | 7 | 24 | 8% | 12% |
| 70 ppm | 5.5 | 8 | 28 | 8% | 11% |
| 80 ppm | 6 | 9 | 32 | 8% | 11% |
| 90 ppm | 7 | 10.5 | 36 | 8% | 12% |
| 100 ppm | 7.5 | 11.5 | 40 | 8% | 12% |
The last two columns are the point: the ppm figures climb steeply while the ratio holds steady — roughly 8% of CYA as the floor, 11–12% as a comfortable target, 40% for shock. The exception is the top row: below about CYA 30 the ratio would call for less chlorine than any pool should run, so an absolute floor of 2 ppm minimum and 4 ppm target takes over.
So a pool at CYA 30 is well sanitized at 4 ppm, while a pool at CYA 90 is scraping the floor at 7 and genuinely needs about 10. Both readings would look "high" to someone raised on the 1–3 ppm rule. Only one of them is protected.
The three numbers worth memorizing
7.5% of CYA — the minimum. Below this, algae has an opening. It's a floor to stay above, not a level to aim for, because chlorine falls throughout the day and you don't want to spend the evening underneath it. At very low CYA the percentage stops being the binding constraint — no outdoor pool should sit under about 2 ppm regardless of what the ratio permits.
~11.5% of CYA — the target. Enough headroom that a hot sunny day or a pool party doesn't push you under the minimum before you next test. This is what the chlorine calculator doses toward.
40% of CYA — shock level. The concentration that clears an algae bloom, held until the water passes the overnight test. It scales with CYA like everything else, which is why a heavily stabilized pool is so punishing to recover — see the SLAM calculator.
You'll be told that high CYA "locks" your chlorine and that only draining or a special product releases it. There is no lock. What's actually happening is far more ordinary: at high CYA, the chlorine you're adding was never enough to be effective at that CYA level. A pool at CYA 100 needs 7.5 ppm just to reach its minimum — dose it to 3 and it behaves exactly like a pool with no chlorine, because functionally it has none. The fix isn't a bottle. It's either more chlorine or less CYA, and the table above tells you which.
Pucks are a loan against your water
Almost nobody over-doses stabilizer directly. It arrives inside stabilized chlorine — trichlor pucks and dichlor granules both carry CYA, and it stays behind when the chlorine is consumed. The stabilizer calculator covers where a high reading comes from and what to do about it; the part that belongs here is the arithmetic, which is quietly brutal:
| Chlorine source | CYA added per 10 ppm of FC | After a month at 2 ppm/day |
|---|---|---|
| Trichlor pucks | +6.1 ppm | +37 ppm CYA |
| Dichlor granules | +9.1 ppm | +55 ppm CYA |
| Liquid chlorine / bleach | none | none |
| Cal-hypo | none (adds calcium instead) | none |
That's the whole story of the pool that started spring at CYA 40, ran a puck floater all season, and hit August at 150 and green. Nothing went wrong; the floater did exactly what it says on the bucket. Pucks are genuinely useful for vacations and slow feed — they're just a loan against your water, and the interest is paid in chlorine you'll need later.
Choosing your CYA level
| Pool | CYA | Reasoning |
|---|---|---|
| Outdoor, liquid chlorine | 30–50 ppm | Enough sun protection before the strength penalty starts to bite |
| Outdoor, salt system | 60–80 ppm | The cell makes chlorine continuously, so a bigger reserve pays off — and protects the cell from working flat out |
| Indoor | 0–20 ppm | No sunlight to defend against, so stabilizer only costs you strength |
| Spa | 0–30 ppm | Hot water burns chlorine fast; most spas are covered, and high CYA plus high temperature is a poor combination |
Higher isn't safer. Every extra 10 ppm of CYA raises the chlorine you must hold forever after, and the only way back down is replacing water.
When CYA is too low
The symptom is unmistakable once you know it: chlorine that reads fine in the evening and near zero the next morning, day after day, with no algae to blame. That's UV doing what UV does. An outdoor pool running at CYA 0–10 is a chlorine subscription — you'll pour in far more over a season than the stabilizer would have cost.
Raising it is straightforward, with two quirks. Granular CYA dissolves slowly, so put the dose in a sock hung in front of a return rather than broadcasting it, and squeeze it occasionally. And count it as dosed immediately but don't retest for a week — undissolved stabilizer reads low, and people who retest at three days routinely double-dose. The stabilizer calculator sizes it for your pool.
One exception: if you chlorinate mainly with pucks or dichlor, you almost certainly don't need to add CYA at all. Check the table above — your chlorine source is already adding it for you.
Does pH still matter?
Yes, but less than the internet suggests — and this is the part that surprises people. In an unstabilized pool, pH strongly controls how much of your chlorine is in the active form, which is where "keep pH at 7.4 or chlorine stops working" comes from. In a stabilized pool, CYA dominates that equilibrium so thoroughly that moving pH across the normal 7.2–7.8 band changes sanitizing strength far less than a 10 ppm swing in CYA does.
The practical upshot: if you're fretting about whether your pH is 7.5 or 7.7 while your FC sits at 3 with CYA at 80, you're polishing the wrong number. Get the ratio right first. pH still earns its place for comfort, for scale and corrosion, and for the saturation index — just not as the main lever on whether your chlorine works.
Testing CYA honestly
The standard CYA test is the one where you fill a tube until a black dot disappears. It works by turbidity, it's read by eye, and it's the least precise test in the kit — ±10 ppm is normal, and two people testing the same sample often disagree. Treat the result as a neighborhood, not a number, and don't chase small changes.
Two things improve it. Read it in daylight rather than shade, holding the tube at waist height and looking straight down. And if your CYA is above the scale, dilute: mix equal parts pool water and distilled water, test that, then double the reading. A kit topping out at 100 will happily read 200 that way — which matters, because "off the scale" and "twice the maximum" call for very different amounts of draining.
Two possibilities, and they lead opposite directions. Below roughly 20 ppm most kits simply read zero, so you may just be low. But CYA can also genuinely disappear over winter: under a cover, certain bacteria convert cyanuric acid into ammonia — and ammonia consumes chlorine at a rate that makes an opening feel impossible. If you're adding chlorine and watching it vanish within the hour, that's the signature. Expect to use a lot of liquid chlorine before it clears, and only restabilize afterwards.
Only water takes it back down
CYA doesn't evaporate, doesn't break down meaningfully in season, and isn't consumed — so the ratio can only be restored from the chlorine side or by replacing water. The math is proportional (100 to 40 means replacing 60% of the water), and the how-to — drain percentages, staged drains, the CYA-reducer products and why not to count on them — lives on the stabilizer calculator. If calcium or salt are also high, the dilution calculator finds the single drain that fixes all of them at once.
The short version
Test CYA before you interpret any chlorine reading. Aim FC at roughly 11% of it, never let it fall below 7.5%, and shock at 40%. Keep CYA where you want it by choosing your chlorine source deliberately rather than letting a puck floater decide. Do that and most of the mysterious pool problems — green water at "good" chlorine, shock that doesn't work, chlorine that won't hold — simply stop happening.