Langelier Saturation Index
The Langelier Saturation Index is a calculated number that predicts whether pool water is in equilibrium with calcium carbonate, tending to corrode, or tending to deposit scale.
Definition
The Langelier Saturation Index is a calculated number that predicts whether pool water is in equilibrium with calcium carbonate, tending to corrode, or tending to deposit scale.
Typical Values: Target: -0.3 to +0.3; 0.0 = perfect balance
In Plain Language
LSI = pH + Temperature Factor + Calcium Factor + Alkalinity Factor - 12.1. A result of 0.0 is perfect balance. Negative values indicate aggressive (corrosive) water; positive values indicate scaling tendency. The target range is -0.3 to +0.3. Temperature is a major factor — the same chemistry produces a higher LSI at 90°F than at 70°F.
Why It Matters
The LSI is the most complete indicator of water balance. It accounts for all the major variables and predicts both equipment damage (corrosion) and surface damage (scaling).
Typical Values
Target: -0.3 to +0.3; 0.0 = perfect balance
Last reviewed: 2026-06-01