Saturation Index (LSI)
The Langelier Saturation Index indicates whether your water is corrosive, balanced, or scale-forming.
Target Range
What is Saturation Index (LSI)?
The Langelier Saturation Index (LSI) is the "master balance" number. It tells you if your water is hungry (corrosive) or overstuffed (scale-forming). It combines pH, Temp, Alkalinity, Calcium, and CYA into one single value representing water balance.
Ideal LSI Range
Perfect balance is 0.00. The acceptable "safe zone" is between -0.30 and +0.30. Stay in this lane to protect your pool equipment and surface.
Pool Water Test Kit
Professional test kit.
Corrosive Water (LSI < -0.3)
When LSI drops below -0.30, water becomes aggressive. It will dissolve calcium carbonate wherever it finds it—stripping your plaster, pitting your concrete, and eating away at heat exchangers. This damage is permanent.
Scaling Water (LSI > +0.3)
When LSI rises above +0.30, water can't hold any more calcium. It precipitates out as white scale on waterline tiles, salt cells, and inside heater tubes, reducing efficiency and flow.
Calcium Hardness Increaser
Increase water hardness.
The Temperature Factor
Temperature plays a huge role. Cold water (winter) lowers LSI, making water more corrosive. Hot water (spa) raises LSI, making it scale-prone. You must adjust your pH and Alkalinity differently in winter vs summer to maintain balance.
Why It Matters
You can have "perfect" pH (7.5) and "perfect" Alkalinity (100) but still destroy your heater if the water temperature is cold or Calcium is too low. LSI looks at the big picture.
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