Most pool advice gives a chlorine target of 1 to 3 ppm. That figure is correct for a pool with no cyanuric acid in it, which describes almost no outdoor pool in the country.
Once there is stabilizer in the water, the correct chlorine level is a percentage of the CYA level, and the pools that fight algae every August are almost always pools running a fixed number against a CYA that has been climbing for years.
The table
| CYA | Minimum FC | Target FC | Shock FC |
|---|---|---|---|
| 0 ppm | 1 | 1–3 | 10 |
| 20 ppm | 2 | 2–4 | 8 |
| 30 ppm | 2 | 3–5 | 12 |
| 40 ppm | 3 | 3–6 | 16 |
| 50 ppm | 4 | 4–6 | 20 |
| 60 ppm | 5 | 5–7 | 24 |
| 70 ppm | 5 | 5–8 | 28 |
| 80 ppm | 6 | 6–9 | 31 |
| 90 ppm | 7 | 7–9 | 35 |
| 100 ppm | 8 | 8–11 | 39 |
| 120 ppm | 9 | 9–13 | 47 |
| 150 ppm | 12 | 12–16 | 59 |
Minimum FC is roughly 7.5% of CYA. Shock level is roughly 40%.
Where the percentage comes from
CYA and hypochlorous acid form an equilibrium. Most of the chlorine in a stabilized pool is bound to cyanurate at any given moment — held in reserve, protected from UV, and not doing any sanitising.
The unbound fraction is what kills algae, and its concentration depends on the ratio of chlorine to CYA rather than on either number alone. Hold the ratio constant and the active chlorine is constant, whatever the absolute figures.
At 7.5%, the active hypochlorous acid concentration is roughly equivalent to an unstabilized pool at 0.05 ppm — enough to stay ahead of algae growth continuously.
This is measured chemistry, not a rule of thumb, and it is why the same 5 ppm reading means an over-chlorinated pool at 30 ppm CYA and a losing battle at 100 ppm.
What the table changes
Test CYA before deciding anything. It is the input to every other number, and most owners test it once a year or never. A pool on tablets gains 40–60 ppm a season — every tablet carries it — so last year's figure is not this year's.
Shock is a level, not a dose. "Shocking" means raising free chlorine to the shock column and holding it there until the chlorine stops disappearing overnight. A single bag into a 100 ppm CYA pool gets nowhere near 39 ppm, which is why shocking a high-CYA pool feels like it does nothing. It literally does not.
High CYA is expensive, permanently. Running a pool at 100 ppm CYA means holding 8–11 ppm free chlorine every day of the season instead of 4–6. That is roughly twice the chlorine, forever, and it shows up as a doubled line in the seasonal budget.
The overnight test
The only real measure of whether a pool is winning:
Test free chlorine at dusk. Test again at dawn, before sun hits the water. A healthy pool loses 1 ppm or less overnight. More than that means something is consuming chlorine — algae, usually invisible at this stage — and the pool needs the shock level, not the target level.
Run this any time a pool looks slightly dull. It catches an algae bloom about a week before it is visible, and a week early is the difference between a day of shocking and a green pool.
The right CYA to run
30–50 ppm for a manually chlorinated outdoor pool. Enough protection from UV, low enough that the chlorine requirement stays modest.
60–80 ppm for a salt pool. Cells generate continuously at low output and benefit from more reserve, and the manufacturers specify it.
0–20 ppm indoors. There is no UV to protect against, and stabilizer only makes the chlorine less effective.
Above 100 ppm, no chlorine level is comfortable to swim in and the pool needs water replaced.
The two tests that are worth buying properly for
CYA and free chlorine, and neither is well served by strips. CYA strips commonly read 30–50 ppm out. FAS-DPD titration for chlorine resolves to 0.2 ppm and does not bleach at high levels — which matters, because a bleached DPD test reading zero in a heavily chlorinated pool is the one genuinely dangerous failure mode in pool testing.
A drop-based kit costs $70–90 and lasts years. Against a $500 season of chemicals, it is the cheapest accuracy available.