FC / CYA CHART: free chlorine levels by stabilizer

All values in ppm

CYA	Minimum FC	Target FC	Shock (SLAM)	Mustard algae
0	1	3–5	10	12
10	1	3–5	10	12
20	2	4–6	10	12
30	2	4–6	12	18
40	3	5–7	16	24
50	4	6–8	20	30
60	5	7–9	24	36
70	5	7–9	28	42
80	6	8–10	32	48
90	7	9–11	36	54
100	8	10–12	40	60

Green rows: the usual CYA range for a chlorine pool. Salt pools often run CYA 60–80 to protect the chlorine the cell makes.

Never let FC fall below the minimum, or algae can start. Shock: raise FC to the shock level and hold it until the water is clear, CC ≤ 0.5, and it loses ≤ 1 ppm overnight.

Why chlorine targets depend on CYA

Cyanuric acid shields chlorine from sunlight, but it does it by binding most of the chlorine and holding it in reserve. The more CYA, the less chlorine is actively sanitizing at any given test reading. That's why 3 ppm keeps one pool spotless and lets another turn green: it depends on the CYA.

The rule behind the chart: keep free chlorine at at least about 7.5% of the CYA level. To shock (SLAM), raise it to about 40% of CYA, and about 60% for mustard algae.

When CYA gets too high

Pools on trichlor tabs or dichlor build CYA every week. Past about 80–100 ppm, the chlorine needed becomes impractical and shocking takes enormous doses. The fix is a partial drain. The drain tab in the chemical calculator tells you how much.

To work out a shock dose for a specific pool, use the shock calculator.