What is the Langelier Saturation Index?
The Langelier Saturation Index (LSI) is a calculation that predicts whether pool water will deposit calcium carbonate scale or dissolve it from pool surfaces. Developed by Dr. Wilfred Langelier in 1936, it's become the standard for measuring water balance in pools and spas.
In simple terms: LSI tells you if your water is "hungry" (corrosive) or "full" (scale-forming).
Why LSI Matters More Than Individual Readings
You can have perfect pH, perfect alkalinity, and perfect calcium hardness—and still have water that damages your pool. That's because water balance is about the relationship between these factors, not the individual numbers.
Corrosive Water (Negative LSI)
When LSI is below -0.3, water is undersaturated with calcium carbonate and will try to obtain it from:
- Plaster and pebble surfaces (etching)
- Grout and tile (dissolution)
- Metal equipment (corrosion)
- Heater elements (pitting)
Scale-Forming Water (Positive LSI)
When LSI is above +0.3, water is oversaturated and will deposit calcium carbonate on:
- Tile surfaces (white scale)
- Salt cells (reduced efficiency)
- Heater elements (insulation effect)
- Plumbing (flow restriction)
The LSI Formula
LSI = pH + TF + CF + AF - 12.1Where:
- pH = Actual pH measurement
- TF = Temperature Factor
- CF = Calcium Factor (log of calcium hardness)
- AF = Alkalinity Factor (log of total alkalinity, adjusted for CYA)
Interpreting Results
| LSI Value | Status | Action |
|---|---|---|
| Below -0.5 | Severely corrosive | Immediate correction needed |
| -0.5 to -0.3 | Slightly corrosive | Adjust soon |
| -0.3 to +0.3 | Balanced | Ideal range |
| +0.3 to +0.5 | Slightly scaling | Adjust soon |
| Above +0.5 | Severely scaling | Immediate correction needed |
How Temperature Affects LSI
This is the factor most pool pros underestimate. Warmer water holds less dissolved calcium, pushing LSI positive (toward scaling).
Example:
A pool balanced at 70°F in spring can become scale-forming at 90°F in summer—with no other changes to chemistry.
| Temperature | TF Factor | Effect on LSI |
|---|---|---|
| 50°F | 0.3 | Very negative |
| 70°F | 0.5 | Slightly negative |
| 80°F | 0.6 | Neutral |
| 90°F | 0.7 | Slightly positive |
| 104°F | 0.9 | Very positive |
Pro tip: Always measure water temperature, especially for spas and heated pools.
Adjusting LSI
To Raise LSI (Water is Corrosive)
- Raise pH - Most direct impact
- Raise calcium hardness - Add calcium chloride
- Raise alkalinity - Add sodium bicarbonate
To Lower LSI (Water is Scaling)
- Lower pH - Add muriatic acid
- Dilute with fresh water - Lowers calcium and alkalinity
- Lower alkalinity - Carefully, as this affects pH stability
LSI and Different Pool Types
Plaster Pools
Most sensitive to LSI. Corrosive water will etch and roughen the surface, while scaling water causes white deposits and rough texture.
Fiberglass Pools
More tolerant of LSI variations but corrosive water can cause fading and chalking of the gelcoat.
Vinyl Liner Pools
Least affected by LSI directly, but corrosive water damages metal components and scaling clogs equipment.
Salt Pools
Salt cells are extremely sensitive to scaling. Keep LSI below +0.3 to extend cell life.
Using the PoolOps LSI Calculator
Our free LSI calculator instantly calculates LSI from your readings and provides specific recommendations for correction.
PoolOps also:
- Calculates LSI at every service stop
- Tracks LSI trends over time
- Alerts you to problems before damage occurs
- Accounts for temperature automatically