What is the Langelier Saturation Index (LSI)?

The Langelier Saturation Index (LSI) is a calculated number used to predict the calcium carbonate stability of water. Developed by Leon Langelier in 1936, it indicates whether water will precipitate, dissolve, or be in equilibrium with calcium carbonate. In practical terms, especially for pool owners and industrial water system operators, it tells you if your water is likely to scale (deposit minerals) or be corrosive (eat away at surfaces).

How to Use the LSI Calculator

To get an accurate LSI reading, you need five key water chemistry parameters: pH, Water Temperature, Calcium Hardness, Total Alkalinity, and Total Dissolved Solids (TDS). Simply input these values into the fields above and click calculate. The tool applies the complex logarithmic formula to provide a single decimal value.

Interpreting Your Results

The ideal LSI range for swimming pools and most water systems is between -0.30 and +0.30.

• Negative LSI (< -0.30): The water is undersaturated and corrosive. It will seek to satisfy its hunger for minerals by dissolving calcium from pool plaster, grout, and metal components.
• Balanced LSI (-0.30 to +0.30): This is the "sweet spot." The water is chemically stable and unlikely to cause damage or scale.
• Positive LSI (> +0.30): The water is supersaturated. Excess calcium will precipitate out of the water, leading to cloudy water and scale deposits on pipes, tiles, and equipment.

Frequently Asked Questions

Why does temperature affect LSI? As water temperature rises, calcium becomes less soluble. This means warmer water is more likely to form scale than colder water with the exact same chemical levels.

How often should I check LSI? For residential pools, checking monthly is usually sufficient, though you should check more frequently during extreme weather changes or high usage periods.