LSI / Water Balance Calculator
Enter your pH, calcium hardness, alkalinity, temperature, CYA, and TDS — we'll calculate your LSI and tell you if your water is corrosive, balanced, or scale-forming.
LSI
Safety first
- Never mix chemicals together.
- Always add the chemical to the water — never water to the chemical.
- Wear gloves and eye protection when handling pool/spa chemicals.
- Keep chemicals out of reach of children and pets.
- Always follow the product label — it overrides any guidance here.
How it's calculated
LSI = pH + temperature factor + calcium factor + alkalinity factor − TDS factor. Alkalinity is corrected for CYA's buffering effect first.
Interpreting your LSI
-0.3 to +0.3 = balanced. Below -0.3 = corrosive. Above +0.3 = scale-forming.
Frequently asked questions
What is the LSI and why should I care?
The Langelier Saturation Index combines pH, calcium hardness, alkalinity, temperature, and TDS into one number that predicts whether your water will dissolve (corrode) or deposit (scale) calcium carbonate — useful for protecting plaster, grout, heaters, and metal fittings.
My LSI is negative — what does that mean?
A negative LSI (below -0.3) means your water is corrosive — it will tend to pull calcium and metals out of your pool surfaces and equipment. Raise pH, TA, or calcium hardness to correct it.
My LSI is positive — what does that mean?
A positive LSI (above +0.3) means your water is scale-forming — it will tend to deposit calcium carbonate on surfaces and in your heater. Lower pH, TA, or calcium hardness to correct it.
Is LSI only for plaster pools?
No — vinyl and fiberglass pools don't need to worry about etching plaster, but LSI still predicts scale buildup on heaters and equipment, so it's useful for every pool type.