Pool LSI guides
The Langelier Saturation Index combines five water chemistry measurements — pH, temperature, calcium hardness, total alkalinity, and cyanuric acid — into a single number that tells you whether your water is corrosive, scale-forming, or balanced. A positive LSI means scale is likely. A negative LSI means the water is hungry for calcium and will pull it from wherever it can find it — your plaster, your grout, your equipment. Individual parameters can look fine while the LSI sits in dangerous territory, which is why most pool problems that "don't make sense" start making sense once you look at LSI.
Why LSI Is the Pool Chemistry Number Most Owners Ignore
Corrosive water etches plaster and attacks metal. Scale fouls heaters and salt cells. What bad LSI actually costs you.
LSI and Saltwater Pools: Why SWG Owners Need to Watch Water Balance More Closely
Every hour a salt chlorine generator runs, it pushes pH upward. Combined with summer heat, this makes SWG pools uniquely prone to scale — including on the cell itself.
Why Two LSI Calculators Can Give You Different Answers
Run the same numbers through two calculators and get different LSI results. Here are the four formula differences that cause it — and which answer to trust.
CSI vs. LSI: Are They the Same Thing?
Pool apps use both terms. They measure the same thing — but the name doesn't tell you which formula is actually being used.
LSI by Pool Surface Type: Plaster, Vinyl, and Fiberglass
Same formula, different stakes. What corrosive or scale-forming water actually costs each surface type — and how the calcium hardness targets differ.
Why Your LSI Changes After It Rains
Rain adds pure, acidic water that dilutes calcium, alkalinity, and pH simultaneously. Here's what happens to all four LSI factors — and when to test.
How CYA Changes Your LSI Calculation
Most LSI calculators ignore CYA. When they account for it, high stabilizer levels lower effective alkalinity — shifting LSI negative by more than you'd expect.
PoolChem Tracker calculates your LSI on every reading
Enter your five water parameters and get a full LSI calculation using temperature and CYA-corrected alkalinity — not the simplified formula most apps use.
