What is the ideal FC to CYA ratio?

Keep free chlorine at 10–15% of your CYA for everyday maintenance, and never let it fall below 7.5%. So at a CYA of 40, your target free chlorine is about 4–6 ppm and your hard floor is 3 ppm. To clear active algae you go much higher (40% of CYA), but that's a temporary shock level, not a daily target.

Why isn't my chlorine working even though the level looks fine?

Almost always because your CYA is high and your free chlorine, while it looks normal, is below the ratio. CYA binds chlorine. At CYA 80, a free chlorine of 2 ppm is effectively powerless because too little of it is in the active form. Raise free chlorine to at least 7.5% of your CYA and it starts sanitizing again.

Not by itself. In sunny climates a CYA of 70–80 is smart. It makes chlorine last far longer against UV. CYA only becomes a real problem above about 100, where the chlorine you'd need to keep the ratio gets impractical. And no chemical lowers CYA; the only way down is a partial drain and refill with fresh water.

Is it safe to swim at high free chlorine?

Yes. Free chlorine is safe to swim in up to about 15 ppm at any CYA above 30: the CYA buffers it. People worry about a 'high' chlorine number when the real risk is the opposite: chlorine too low for the CYA, which lets algae and bacteria move in.

The FC/CYA Ratio: Why Your Chlorine Number Means Nothing on Its Own

Here’s the punchline first, because it fixes more pools than any single product: a free-chlorine reading is meaningless until you compare it to your CYA. Chlorine should sit at 7.5% of your CYA at the bare minimum, 10–15% for a healthy target, and 40% to clear active algae. So “my free chlorine is 3” isn’t good news or bad news. It depends entirely on whether your CYA is 20 or 80. Miss this one relationship and you’ll chase a green pool in circles while every test looks “fine.”

Let me untangle it, because once it clicks, most of pool chemistry stops feeling like guesswork.

CYA is chlorine’s sunscreen, and its handcuffs

CYA (cyanuric acid, aka stabilizer) does one genuinely useful job: it shields free chlorine from the sun. Without it, UV burns off your chlorine in hours. With it, chlorine lasts all day. Great deal, right?

The catch: CYA also binds chlorine. At any moment, most of your free chlorine is parked on CYA molecules, held in reserve, not actively sanitizing. Only a small active fraction is out there killing algae and bacteria. The more CYA you have, the more total chlorine you need to keep that active fraction high enough to matter. That’s the whole tension in one sentence: CYA protects your chlorine and muffles it, so the right chlorine level scales with your CYA.

The three numbers that run your pool

Everything flows from three percentages of your CYA:

Minimum free chlorine = CYA × 0.075 (7.5%). Below this, so little chlorine is active that algae can bloom within days.
Target free chlorine = CYA × 0.10 to 0.15 (10–15%). The healthy everyday band.
Shock (SLAM) free chlorine = CYA × 0.40 (40%). The temporary level to clear an active algae bloom, not a daily target.

Here’s the full lookup, straight from the TroubleFreePool PoolMath methodology:

CYA (ppm)	Minimum FC	Target FC	Shock (SLAM) FC
20	1.5	2–3	8
30	2	3–5	12
40	3	4–6	16
50	4	5–8	20
60	5	6–9	24
70	5	7–11	28
80	6	8–12	32
90	7	9–14	36
100	8	10–15	40

Notice what this means: at CYA 80, a free chlorine of 2 ppm (a number a pool store might cheerfully call “fine”) is actually below the minimum. Your chlorine is real, it’s just handcuffed to the CYA and barely sanitizing. That single misread is the most common reason a pool goes green while the strip looks okay.

The two failure modes

Chlorine too low for the CYA. This is the dangerous one. Too little active chlorine, and algae gets a foothold in a matter of days. If your pool keeps going green despite “normal-looking” chlorine, recompute your target as a percentage of your actual CYA. I’d bet you’re under it.

Chlorine above the target band. This one gets worried about far more than it deserves. Free chlorine up to about 15 ppm is safe to swim in at any CYA over 30. Overshooting your target doesn’t hurt anything except your wallet. It’s just chlorine you didn’t need to buy. Not a safety problem; a budgeting one.

”High CYA is bad”: mostly a myth

You’ll hear “your CYA is too high” as a reflex diagnosis. It’s usually wrong. In high-UV places (Phoenix, Vegas, Miami), a CYA of 70–80 is actually smart, because it makes your chlorine last dramatically longer. Indoors or in low sun, CYA of 30 or less is plenty. The only time “lower your CYA” is the right call is when it’s near 100 and above and you’re fighting algae despite normal-looking numbers, because the ratio math gets impractical up there. And remember: nothing chemical lowers CYA. The only way down is draining some water and refilling.

Put the ratio to work

This ratio is the engine behind two other things worth reading: it’s exactly the target you hold during a green-to-clean SLAM, and holding it in the healthy band is the entire secret to never seeing algae again. Learn this one relationship and you’ve learned the load-bearing idea in pool chemistry.

Or let me do the arithmetic for you. AL GEE reads your CYA and tells you your exact target and dose, every time, so you never have to squint at a table again. Free to start, no card required.

AL GEE is Azoolify’s AI pool expert. These ratios are canonical pool chemistry: the TroubleFreePool PoolMath chart, derived from equilibrium chemistry, not from any chemical brand’s dosing table (those tend to run high, because high sells more chlorine). When in doubt, aim low; you can always add more.