Pool chemistry

Six numbers keep a pool clear and safe. Here's what each one does and exactly how much to add.

Pool water chemistry looks complicated because test kits report half a dozen numbers. In practice, two of them do most of the work: free chlorine keeps the water sanitary, and pH decides how well that chlorine works and whether the water is comfortable. Total alkalinity steadies pH, cyanuric acid (stabilizer) protects chlorine from sunlight, and calcium hardness protects plaster surfaces.

Start with your pool’s volume, since every dose depends on it. Then test and adjust one thing at a time: alkalinity first, then pH, then chlorine.

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What to test, how often, and what the numbers mean.

Pool Water Chemistry for Beginners: What Each Test Number MeansWhat free chlorine, pH, alkalinity, CYA, calcium hardness and combined chlorine actually tell you, which ones matter most, and what order to fix them in.Read the guide (4 min)

Chlorine and shocking

Keeping free chlorine where it needs to be for your stabilizer level.

How to Shock a Pool (and How Much Shock You Actually Need)How to shock a pool so it actually works. Find the right chlorine level for your stabilizer, choose a product, and hold the level until the job is done.Read the guide (4 min)

pH and alkalinity

The buffer pair that decides whether your pH drifts.

How to Lower (or Raise) Pool pH SafelyWhy pool pH drifts up, how to bring it down with muriatic or dry acid, how to raise it, and why your alkalinity decides how much acid you actually need.Read the guide (3 min)

Stabilizer and salt

CYA protects chlorine from the sun; salt feeds a chlorine generator.

Cyanuric Acid (Pool Stabilizer): How Much You Need and Why It MattersStabilizer stops sunlight from destroying chlorine, but it also raises the chlorine level your pool needs. How CYA works, good levels, and how to raise or lower it.Read the guide (3 min)