How Chlorine and CYA Actually Work (the Science Behind the FC/CYA Chart)
Chlorine's real sanitizer is hypochlorous acid (HOCl). CYA binds most of your free chlorine into a protected reserve and releases HOCl a little at a time, so only a tiny fraction is active at any moment. The active amount tracks the ratio of free chlorine to CYA (HOCl is proportional to FC/CYA — 'Falk's ratio'). That single fact is why the FC/CYA chart scales chlorine with CYA, why ~7.5% of CYA is the algae-prevention floor, and why 'chlorine lock' is really just CYA too high. Keep FC at its CYA target and the active chlorine takes care of itself.
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.
- Minimum ≈ 7.5% of CYA — this holds active HOCl near about 0.02 ppm, the level that reliably prevents algae in a balanced pool. (CYA 50 → about 3.75 ppm floor.)
- Target ≈ 11–12% of CYA — a comfortable working level above the floor.
- SLAM ≈ 40% of CYA — enough active chlorine to kill an established bloom faster than it grows.
See the full FC/CYA chart for every CYA level.
Here’s that ratio drawn — the floor and the target band climbing with CYA is Falk’s proportionality, nothing more:
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.
Run the ratio for your pool
The same chemistry, computed: enter your CYA (and your current FC if you know it) and get the floor, target, and SLAM level.
Straight off Trouble Free Pool's FC/CYA chart (minimum ≈ 7.5% of CYA, SLAM ≈ 40%), snapped to the nearest published row — the chart table stays the source of truth. Exact doses for your gallons: the chemical calculator. CYA way over 100? More chlorine isn't the fix — lowering CYA is.
Frequently asked
Why do I need more chlorine when my CYA is higher?
Because CYA binds most of your chlorine into a reserve, and only the small unbound fraction — hypochlorous acid, HOCl — actually sanitizes. The active HOCl tracks the ratio of free chlorine to CYA, so to keep the same sanitizing power when CYA goes up, free chlorine has to go up with it. That's the whole reason the FC/CYA chart isn't one fixed number: a reading of 3 ppm is strong at CYA 30 and near-useless at CYA 90.
What is hypochlorous acid (HOCl) and why does it matter?
HOCl is the active, germ-killing form of chlorine in water. The other forms — hypochlorite ion (OCl-) and chlorine bound to CYA (chlorinated cyanurates) — are orders of magnitude weaker sanitizers. Your 'free chlorine' test measures the available pool, but it's the HOCl fraction that does the work, and CYA controls how much of it is active at once.
Where does the 7.5%-of-CYA minimum come from?
It's not a rule of thumb someone guessed — it's derived from chemistry. Richard Falk ('chem geek' on Trouble Free Pool) worked out that keeping free chlorine at about 7.5% of your CYA holds active HOCl near the level (about 0.02 ppm) that reliably prevents algae in a balanced pool. So at CYA 50, about 3.75 ppm free chlorine is the floor; below it, algae gets an opening even though the number 'looks fine.'