Free Chlorine Target Calculator

The FC/CYA ratio is the relationship between free chlorine (FC) and cyanuric acid (CYA) that keeps chlorine effective. Because CYA binds chlorine, the right free-chlorine target scales with your CYA level. The widely referenced guideline is to keep free chlorine at roughly 7.5% of your CYA — for example, about 4 ppm FC at 50 ppm CYA.

It depends on your CYA. At 30 ppm CYA, target 2–3 ppm free chlorine; at 50 ppm, about 4 ppm; at 80 ppm, around 6 ppm. A single fixed number like '1–3 ppm' is misleading once CYA is present. Enter your CYA above to get a target range for a manual or saltwater pool.

CYA shields chlorine from UV sunlight by binding most of it into a reserve, releasing it slowly. The trade-off is that the higher your CYA, the more total free chlorine you must carry for the same amount of active sanitizer. That's why a pool at 100 ppm CYA needs far more free chlorine than one at 30 ppm to stay algae-free.

Shocking (or a SLAM) raises free chlorine to roughly 40% of the CYA level to break down chloramines and kill algae — for example, about 20 ppm FC at 50 ppm CYA. The calculator shows the shock target alongside your normal range. Always shock with an unstabilized chlorine source so you don't add more CYA.

Saltwater pools follow the same FC/CYA principle, but because the generator produces chlorine continuously, many operators run a slightly lower target — around 5% of CYA — and sometimes keep CYA a bit higher (60–80 ppm) for UV protection. The calculator's saltwater mode reflects this lower maintenance target.