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Free chlorine and CYA: the ratio that matters
The most common reason a chlorine pool goes cloudy or green isn't too little chlorine in absolute terms — it's too little chlorine relative to the stabilizer. Once you understand the link between free chlorine (FC) and cyanuric acid (CYA), pool care stops being guesswork.
What cyanuric acid does
Cyanuric acid is chlorine stabilizer. Outdoors, unprotected chlorine is destroyed by UV in a couple of hours. CYA acts like sunscreen: it holds a reserve of chlorine and releases it slowly, dramatically slowing UV loss. Without some CYA, an outdoor pool can't hold chlorine through a sunny afternoon.
But there's a catch. CYA also binds most of your chlorine into that reserve, leaving only a small fraction active at any moment. The more CYA you have, the more total FC you must carry to keep enough active chlorine to sanitize.
This is why a single "ideal chlorine" number is wrong. 3 ppm FC is plenty at CYA 20 and dangerously weak at CYA 80. The correct FC depends entirely on your CYA.
The FC/CYA target ratio
The modern, widely-used approach (popularized by Trouble Free Pool) ties FC to CYA as a percentage. As a rule of thumb, keep FC at roughly 7.5% of CYA as the minimum and around 11–12% as the target:
| CYA (ppm) | Min FC | Target FC |
|---|---|---|
| 30 | 2 | 4–6 |
| 40 | 3 | 5–7 |
| 50 | 4 | 6–8 |
| 70 | 5 | 8–10 |
Let FC fall below the minimum for your CYA and algae gets a foothold — even though the number might look "normal" by the old chart.
Trichlor pucks and dichlor shock each add CYA every time they add chlorine. Use them all season and CYA creeps up, your required FC climbs with it, and eventually you can't keep up — the classic mid-summer green pool. The only way to lower CYA is to dilute with fresh water.
Splash sets your FC target from your CYA
Enter your test readings and Splash gives you the right free-chlorine target for your exact stabilizer level — plus the dose to get there and a full balance check. Free to download.
Sources
- Trouble Free Pool — FC/CYA relationship and recommended levels
- CDC / pool operation guidance (chlorine and cyanuric acid fundamentals)
Ratios are guidelines for residential pools; indoor and commercial pools follow different rules.