Cyanuric Acid & Chlorine: the FC/CYA Relationship Explained

Cyanuric acid (CYA) — also sold as stabilizer or conditioner — and chlorine are a package deal in any outdoor pool. Get the balance right and your chlorine lasts all day; get it wrong in either direction and you’ll either burn through chlorine by noon or watch it stop working entirely. Here’s how the two relate, in plain English.

Why chlorine needs cyanuric acid

Sunlight is brutal on chlorine. Ultraviolet light breaks free chlorine down fast — studies put it at roughly half gone within about 17 minutes of direct sun, and an unprotected outdoor pool can lose 50–90% of its chlorine in just a few hours. Without protection, you simply can’t hold a sanitizer level through an afternoon.

Cyanuric acid fixes this. It forms a loose, reversible bond with chlorine that acts like sunscreen: it shields the chlorine from UV while still releasing it to kill germs and algae. With CYA in the water, chlorine lasts roughly 3–5× longer. That’s why CYA is essential for outdoor pools — and pointless (even discouraged) for indoor pools and hot tubs, which get no sun.

The catch: too much CYA “locks” your chlorine

The same bond that protects chlorine also slows it down. As CYA climbs, a smaller and smaller fraction of your free chlorine is in its active form at any moment. Push CYA too high — above roughly 80–100 ppm — and chlorine becomes so sluggish that the water can look “in range” on a test yet sanitize poorly. This is over-stabilization, and it’s a common cause of cloudy or algae-prone water in pools that test fine for chlorine.

Because nothing chemical removes CYA, the only way to bring it back down is to drain and dilute. So the goal is to keep CYA in a sweet spot, not to chase it up.

The FC/CYA ratio — the number that ties them together

The practical takeaway is that your correct chlorine level depends on your CYA, not on a single “2–4 ppm” rule. The higher your CYA, the more free chlorine you need to keep the same sanitizing power. A widely used guide:

• Minimum free chlorine ≈ 7.5% of CYA (never let it drop below this)
• Target free chlorine ≈ 11–12% of CYA for everyday operation
• Shock level ≈ 40% of CYA to clear algae

So in practice:

• CYA 30 → minimum ~2 ppm, target ~4–6 ppm
• CYA 50 → minimum ~4 ppm, target ~6–8 ppm
• CYA 80 → minimum ~6 ppm, target ~9–11 ppm

Putting it to work

Aim for 30–50 ppm CYA in a traditional chlorine pool, or 60–80 ppmin a saltwater pool (salt systems make chlorine at a lower rate, so the extra buffer helps). Then hold your free chlorine at the target for that CYA level. If your CYA is low, your chlorine will keep vanishing in the sun; if it’s very high, raise your chlorine target or dilute the CYA back down. Our calculators do this math for you — the chlorine calculator reads your CYA and recommends the right target automatically, and the stabilizer calculator tells you how much CYA to add or how much water to drain.