Pool chemistry 101
The handful of numbers that keep your water safe and clear, in plain English.
- pH
- How acidic or basic the water is, on a 0–14 scale. Pools target 7.4–7.6. Off-balance pH stings eyes, corrodes equipment, and makes chlorine work poorly.
- Total alkalinity (TA)
- A measure of the water’s buffering capacity — pH’s “shock absorber.” Target ~60–120 ppm. Too low and pH bounces; too high and pH drifts up and water clouds.
- Free chlorine (FC)
- The active sanitizer available to kill germs and algae. Your target rises with CYA (about 7.5% of CYA).
- Combined chlorine (CC)
- Used-up chlorine bound to contaminants — the irritating, “chlorine smell” kind. Above 0.5 ppm means it’s time to shock.
- Cyanuric acid (CYA)
- Stabilizer or conditioner — “sunscreen” for chlorine. It protects chlorine from sunlight, but too much weakens sanitizing power. Target 30–50 ppm (60–80 for saltwater).
- Calcium hardness (CH)
- Dissolved calcium. Too low etches plaster and corrodes metal; too high scales surfaces and clouds water. Plaster pools target 250–450 ppm.
- Salt
- Sodium chloride that feeds a salt chlorine generator (SWG). Typical target ~3,200 ppm; check your cell’s manual.
- LSI (Langelier Saturation Index)
- A single number combining pH, temperature, calcium, and alkalinity that tells you whether water is balanced, scale-forming, or corrosive. Aim for roughly −0.3 to +0.3.
- Shock
- Raising free chlorine high (about 40% of CYA) to kill algae and burn off contaminants, then holding it there until the water clears.
- ppm
- Parts per million — the unit for most pool readings (1 ppm ≈ 1 milligram per liter).