Langelier Saturation Index (LSI) Formula
The Formula
| Symbol | Description | Unit |
|---|---|---|
pH | Measured pH of the pool water | pH units |
TF | Temperature Factor — from lookup table by water temperature | factor |
CF | Calcium Factor — from lookup table by calcium hardness level | factor |
AF | Alkalinity Factor — from lookup table by total alkalinity level | factor |
12.1 | Constant representing the saturation pCa + pAlk at equilibrium | constant |
Worked Example
Pool conditions: pH 7.4, temp 80°F, calcium hardness 300 ppm, total alkalinity 100 ppm.
- pH = 7.4
- TF at 80°F = 0.6 (from table)
- CF at 300 ppm = 1.8 (from table)
- AF at 100 ppm = 1.9 (from table)
- LSI = 7.4 + 0.6 + 1.8 + 1.9 − 12.1 = 1.6
Actually: let's use typical table values. - pHs (saturation pH) ≈ 9.6 for these conditions - LSI = 7.4 − 9.6 = −2.2?
Using the simplified additive form: - LSI = pH + TF + CF + AF − 12.1 - LSI = 7.4 + 0.6 + 1.9 + 2.0 − 12.1 = −0.2
An LSI of −0.2 is within the acceptable range (−0.3 to +0.5). This water is very slightly corrosive but within normal operating parameters.
How This Formula Works
The Langelier Saturation Index (LSI) measures whether pool water is corrosive, balanced, or scaling. A score of 0 means the water is perfectly balanced. Negative LSI means the water is corrosive — it will dissolve plaster, grout, and metal. Positive LSI means the water is scale-forming — calcium carbonate will precipitate on surfaces.
The three lookup factors: - Temperature Factor (TF): higher temperature → higher TF (water more prone to scaling at heat) - Calcium Factor (CF): higher calcium hardness → higher CF - Alkalinity Factor (AF): higher alkalinity → higher AF
All three factors are found from standard LSI lookup tables, which convert each parameter into a logarithmic index value.
Limitations & Notes
The LSI is a thermodynamic equilibrium model — it predicts tendency, not certainty. Other factors including TDS, flow rate, surface type, and chemical inhibitors affect actual scaling or corrosion in practice. The LSI is most useful as a diagnostic check and should be calculated at the actual pool water temperature.
Last reviewed: 2026-06-01