How Chlorine and CYA Actually Work (the Science Behind the FC/CYA Chart)

The FC/CYA chart can feel like magic numbers someone made up. It isn’t. It falls straight out of one piece of chemistry — and once you see it, chlorine lock, “green but my levels are fine,” and “why do I keep needing more chlorine” all stop being mysteries.

There are two kinds of chlorine, and only one works

When chlorine dissolves in water it becomes hypochlorous acid (HOCl) — the active form that actually kills algae and bacteria — and hypochlorite ion (OCl⁻), which is a far weaker sanitizer. Your free-chlorine (FC) test counts the available chlorine, but it’s the HOCl fraction that does the work.

CYA is a reservoir, not a switch

Here’s the part that explains everything. CYA (cyanuric acid) grabs most of your chlorine and holds it in reserve as chlorinated cyanurates — chlorine bonded to CYA, which barely sanitizes on its own. That reserve isn’t wasted: it releases HOCl a little at a time as the active chlorine gets used up. But at any given instant, only a small fraction of your free chlorine is active HOCl. The more CYA, the bigger the reserve, and the smaller that active fraction.

CYA is sunscreen for chlorine — it stops the sun from burning it off in hours. The trade is that it also holds most of your chlorine in reserve, so you have to carry more of it.

The one relationship: active chlorine tracks FC ÷ CYA

Richard Falk (known as “chem geek” on Trouble Free Pool) pinned it down: active HOCl is proportional to the ratio of free chlorine to CYA (HOCl ∝ FC/CYA). That’s the whole engine behind the chart. To keep the same killing power as CYA rises, free chlorine has to rise in proportion — so the target isn’t a fixed ppm, it’s a percentage of your CYA.

Here’s that ratio drawn — the floor and the target band climbing with CYA is Falk’s proportionality, nothing more:

• Target FC band (read it off the chart)
• Minimum FC (≈ 7.5% of CYA)
• Below minimum — algae can grow

What this explains

• “Green but my chlorine’s fine.” A “normal” 3 ppm at CYA 90 is below the 7.5% floor — active HOCl is too low, so algae grows. Same reading, different pool. (Full story: green but levels look fine.)
• “Chlorine lock.” There’s no lock — your CYA is just so high that ordinary chlorine can’t reach an active level. The fix is dilution, not a magic additive. (See CYA too high.)
• Why a commercial pool runs FC 1–3. With little or no CYA, a small FC is nearly all active HOCl. Your backyard pool carries CYA for sun protection, so it needs more FC for the same active chlorine — and that’s completely safe, because the CYA keeps the active level in the same gentle range.

What to do about it

Nothing exotic — just keep free chlorine at the target for your CYA and let the chemistry work. Know your CYA (test it), read your target off the chart, and dose with the calculator. That’s the entire game, and it’s why less is more: get this one ratio right and you don’t need the shelf of bottles that exists to paper over getting it wrong.

If the ratio says your CYA is too low

A pool at or near zero stabilizer burns chlorine off in hours of sun, so the fix is real — but it’s the one chemical where buying too big is the expensive mistake, because CYA does not break down and the only way down is draining. Two sizes cover almost everyone, and the right one depends on whether you’re starting from scratch or topping up. Dissolve either in a sock in the skimmer rather than broadcasting it; undissolved granules on a plaster floor are the most common complaint in this whole category.

Here is why, and it is not the brand’s fault. Cyanuric acid is barely soluble in water — about 2,700 mg per litre, which works out at roughly 45 gallons of water to dissolve a single pound. That is the whole story behind every one-star review in this category. We read the critical reviews of both bags below and eight of ten on each are the same complaint: it will not dissolve. Owners describe a sock at the return jet still holding granules after days, hours of stirring in a five-gallon bucket achieving almost nothing, and broadcast granules settling on the floor, jamming a skimmer basket or clogging a salt cell. A bucket cannot work: two-thirds of a 7 lb bag needs more than two hundred gallons of water to go into solution.

So the dangerous mistake is not under-dosing, it is re-dosing. Add stabilizer, hang it in a sock, squeeze the sock when you walk past, and wait days before you believe a test. Several of the “this product does nothing to my CYA” reviews are owners who added a second dose because the first had not registered yet — and that is exactly how you overshoot a number that only comes back down by draining your pool.

And if the ratio says your CYA is too high, no product fixes it — see CYA too high for the drain math.