What the Langelier Index is
The Langelier Saturation Index (LSI) is a formula that combines four water parameters, pH, temperature, calcium hardness and alkalinity, into a single number that shows the water's tendency: whether it will dissolve scale from surfaces (corrosive/aggressive water) or deposit scale on them (scale-forming water). It's the reference professionals in the industry use to know whether the water is really balanced, beyond looking at each parameter on its own.
Why looking at pH or chlorine alone isn't enough
Two pools can have the perfect pH (7.4) and still behave very differently depending on their calcium hardness, alkalinity or water temperature. A pool with correct pH but very low hardness can still be corrosive and gradually damage tiling, joints or metal fixtures. The LSI is the way to see it all together, the way it actually interacts in the water.
How to read the result
- LSI below -0.3 (corrosive): the water tends to dissolve scale from surfaces and equipment, it can damage tiling, joints, ladders and other fixtures over time.
- LSI between -0.3 and +0.3 (balanced): the target range. The water doesn't attack surfaces or tend to form scale deposits.
- LSI above +0.3 (scale-forming): the water tends to deposit scale, it can show up as whitish deposits on walls, and it also reduces the filtration system's efficiency over time.
You can check each parameter individually in our complete guide to water parameters, or go deeper into pH and PPM if you want to understand the units all this is measured in.