Pool chemistry 101
Updated:
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. The CDC recommends 7.0–7.8, and the MAHC Annex suggests 7.2–7.6 when balancing water. For pools the app uses 7.2–7.8, from the Hayward and Pentair manuals. Chlorine works less well as pH rises, and pipes and equipment corrode faster as it falls.
- Total alkalinity (TA)
- A measure of the water’s buffering capacity, or pH’s “shock absorber.” The MAHC sets 60–180 ppm, the Hayward and Pentair manuals 80–120, which the app uses, and Trouble Free Pool prefers 60–90 for liquid chlorine and salt pools. 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. The CDC floor is 1 ppm in pools, 2 ppm with CYA and 3 ppm in hot tubs. With CYA, Trouble Free Pool’s minimum is 7.5% of your CYA.
- Combined chlorine (CC)
- Used-up chlorine bound to contaminants, the irritating, “chlorine smell” kind. Trouble Free Pool says to look closer above 0.5 ppm, the level at which the app calls for a shock. The MAHC sets 0.4 ppm as the level at which public pools must act, for example by superchlorinating.
- Cyanuric acid (CYA)
- Stabilizer or conditioner, the “sunscreen” for chlorine. It protects chlorine from sunlight, but binds it and slows how fast it works. The MAHC caps it at 90 ppm; the Hayward and Pentair manuals call for 30–50, which the app uses, and Trouble Free Pool suggests 30–60 for liquid chlorine pools and 60–90 for salt pools. The CDC recommends none in hot tubs.
- Calcium hardness (CH)
- Dissolved calcium. Too low etches plaster and corrodes metal; too high scales surfaces and clouds water. The MAHC Annex suggests 200–400 ppm for plaster and 80–200 ppm for vinyl or fiberglass.
- Salt
- Sodium chloride that feeds a salt chlorine generator (SWG). Each cell has its own range: Hayward’s AquaRite wants 2,700–3,400 ppm and Pentair’s IntelliChlor 3,600–4,500 ppm, so check your cell’s manual.
- LSI (Langelier Saturation Index)
- A single number combining pH, temperature, calcium, alkalinity and dissolved solids that tells you whether water is balanced, scale-forming, or corrosive. The MAHC Annex treats −0.3 to +0.3 as balanced.
- Shock
- Raising free chlorine high, to about 40% of CYA in Trouble Free Pool’s method, 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).
Sources
- CDC, Home Pool and Hot Tub Water Treatment and Testing (May 10, 2024). pH range and free chlorine minimums, and no CYA in hot tubs
- CDC, Model Aquatic Health Code, 5th edition (2024), Code Language. Sections 5.7.3.1.3.2 (CYA), 5.7.4.4.1 (alkalinity) and 5.7.4.4.2 (combined chlorine)
- CDC, Model Aquatic Health Code, 5th edition (2024), Annex. Sections 5.7.3.1.3.1 (CYA), 5.7.4.4.3 (calcium) and 5.7.4.6 (pH for balance, calcium by surface, LSI)
- Trouble Free Pool, CYA Chlorine Relationship (Chlorine/CYA chart). Free chlorine minimum and shock level as shares of CYA
- Trouble Free Pool, Pool Chemical Levels: Pool Industry vs TFP (December 21, 2025). Alkalinity and CYA targets
- Trouble Free Pool, Pool Chemistry ABC’s. Combined chlorine above 0.5 ppm calls for a closer look
- Hayward, AquaRite 900 Series Electronic Chlorine Generator manual (092623 Rev D). Salt, pH, alkalinity, stabilizer and calcium ranges
- Pentair, IntelliChlor IC15/IC20/IC40/IC60 Installation and User’s Guide (P/N 520589 Rev. R, 2023). Salt, pH, alkalinity, stabilizer and calcium ranges