Langelier Saturation Index (LSI) Calculator
Find out instantly whether your pool water is corrosive, balanced, or scaling — with South Florida-specific correction guidance for salt pools and high-calcium water.
Enter Your Water Chemistry
LSI Formula (ANSI/APSP-11)
LSI = pH + log₁₀(Ca) + log₁₀(TA) + pTemp − 12.1
pTemp at 84°F = 0.7 · log₁₀(300) = 2.48 · log₁₀(100) = 2.00
Your LSI Score
Water is actively depositing calcium carbonate. Expect cloudy water, white scale on surfaces, clogged filters, and reduced equipment efficiency. Correct promptly.
Recommended Corrections
All parameters: Maintain current levels
Your water is balanced. Test weekly and after heavy rain or heavy bather load.
South Florida Context
Miami tap water from the Biscayne Aquifer is naturally high in calcium (250–350 ppm) and TDS due to the underlying limestone formation. Salt pools further raise pH at the cell plates. After summer rain (pH 5.5–6.5), LSI can swing by 0.4–0.6 points in hours — always retest within 24 hours of significant rainfall.
LSI Interpretation Reference
| LSI Range | Classification | Water Behavior | Primary Risk |
|---|---|---|---|
| Below −0.5 | Corrosive | Aggressively dissolves calcium carbonate | Plaster etching, metal corrosion, eye irritation |
| −0.5 to −0.3 | Slightly Corrosive | Mildly dissolves surfaces over time | Gradual surface wear, equipment damage |
| −0.3 to +0.3 | Balanced ✓ | Neutral — neither deposits nor dissolves | None — maintain this range |
| +0.3 to +0.5 | Slightly Scaling | Mild calcium carbonate deposition | Light scale on surfaces and equipment |
| Above +0.5 | Scaling | Active calcium carbonate precipitation | Cloudy water, heavy scale, clogged filters |
Source: ANSI/APSP-11 Standard for Water Quality in Public Pools and Spas · Taylor Technologies CPO® Handbook
Frequently Asked Questions About LSI
The Langelier Saturation Index (LSI) is a numerical measure of water's tendency to either dissolve or deposit calcium carbonate. Developed by Dr. Wilfred Langelier in 1936, it combines pH, calcium hardness, total alkalinity, and water temperature into a single score. A score of 0 is perfectly balanced; negative values indicate corrosive water; positive values indicate scaling water. The ANSI/APSP-11 standard recommends keeping pool water between -0.3 and +0.3.
Salt chlorine generators raise pH naturally at the cell plates, and South Florida tap water already has high calcium hardness and high TDS (total dissolved solids) due to the limestone aquifer. This combination pushes salt pools toward the scaling side of the LSI. After rain — which is acidic and low in alkalinity — the LSI swings rapidly, causing calcium carbonate to fall out of solution and appear as white cloudy water or dust on pool surfaces.
The ANSI/APSP-11 standard recommends an LSI between -0.3 and +0.3. For South Florida pools — especially salt pools — targeting the slightly negative end of that range (-0.1 to 0.0) provides a buffer against the natural upward pH drift from salt cells and the high-calcium local water supply.
To raise a negative LSI, you need to increase one or more of: pH (target 7.4–7.6 using soda ash), total alkalinity (target 80–120 ppm using sodium bicarbonate), or calcium hardness (target 200–400 ppm using calcium chloride). Always adjust alkalinity before pH, and retest after each addition. Never add multiple chemicals simultaneously.
To lower a positive LSI, reduce pH with muriatic acid or dry acid (target 7.4–7.6), lower alkalinity if above 120 ppm, or reduce calcium hardness by partially draining and refilling with fresh water. In South Florida, calcium hardness is the hardest to control because local tap water is already calcium-rich — regular partial drains (every 2–3 years) are the most effective long-term strategy.
Need Help Balancing Your Pool?
Our certified technicians test and balance pool water on every service visit — including LSI, pH, alkalinity, calcium hardness, and salt levels. Serving Cutler Bay, Palmetto Bay, Pinecrest, Homestead, and Kendall.