Free chlorine and CYA: the ratio that matters
Why "keep chlorine at 1–3 ppm" is the advice that turns pools green.
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.
Gear this guide uses
To measure FC and CYA accurately, then hold the right ratio.
- Measure both FC and CYA with a Taylor K-2006 test kit — guesswork on either number is how the ratio drifts.
- If your CYA is low, set it deliberately with cyanuric acid stabilizer.
- Carry your FC with plain liquid chlorine, which adds no extra CYA.
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Sources
Ratios are guidelines for residential pools; indoor and commercial pools follow different rules.