Chlorine and stabilizer (CYA): how much is enough
Free chlorine targets depend on CYA. Minimum, target and shock levels, and how to dose liquid chlorine, cal-hypo, dichlor and trichlor.
Stabilizer (cyanuric acid, CYA) shields chlorine from sunlight but also holds some of it back. So the right free chlorine (FC) level is a percentage of the CYA level[6]:
- Minimum FC ≈ 7.5% of CYA (CYA 40 → FC 3)
- Target FC ≈ 11.5% of CYA (CYA 40 → FC 4.6)
- Shock (SLAM) = 40% of CYA (CYA 40 → FC 16)
With no CYA at all, keep at least 1 ppm in a pool; CDC advises at least 2 ppm when CYA is used[8].
Dosing
To raise FC by 1 ppm in 10,000 gallons[1]:
- 10% liquid chlorine: 12.8 fl oz (1 gallon raises 10,000 gal by 10 ppm)
- 12% liquid chlorine: 10.7 fl oz
- 65% cal-hypo: 2.0 oz (weight)
- 56% dichlor: 2.4 oz
- 90% trichlor: 1.5 oz
Scale by gallons: double the pool, double the dose. Dichlor and trichlor add CYA every time you use them (about 0.9 ppm CYA for every 1 ppm FC from dichlor, 0.6 from trichlor), which is why CYA creeps up on tab pools.
Combined chlorine and breakpoint
Combined chlorine (CC) = total chlorine − free chlorine. Above 0.5 ppm, raise FC by about 10 × CC to reach breakpoint[1].
Lowering CYA
The only reliable way is draining and refilling. To go from 100 to 50 ppm, replace about 50% of the water.