Langelier Saturation Index (LSI) Formula
The Formula
| Symbol | Description | Unit |
|---|---|---|
pH | Measured pH of the pool water | pH units |
TF | Temperature Factor — from the site's water-balance lookup table, by water temperature | factor |
CHF | Calcium Hardness Factor — from the site's water-balance lookup table, by calcium hardness level | factor |
TAF | Total Alkalinity Factor — from the site's water-balance lookup table, by total alkalinity level (using CYA-corrected TA) | factor |
12.1 | TDS constant for standard pools (~1,000 ppm TDS); use 12.2 for saltwater pools (~3,200 ppm TDS) | constant |
Worked Example
Pool conditions: pH 7.5, water temperature 76°F, calcium hardness 300 ppm, total alkalinity 100 ppm (standard, non-saltwater pool).
Reading directly from the water-balance lookup tables: - TF at 76°F = 0.6 - CHF at 300 ppm = 2.1 - TAF at 100 ppm = 2.0
LSI = pH + TF + CHF + TAF − 12.1 LSI = 7.5 + 0.6 + 2.1 + 2.0 − 12.1 = 0.1
An LSI of 0.1 is within the site's balanced target range (−0.3 to +0.5) — this water is essentially in equilibrium, neither notably corrosive nor scale-forming.
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 Hardness Factor (CHF): higher calcium hardness → higher CHF - Total Alkalinity Factor (TAF): higher alkalinity → higher TAF
All three factors are read from this site's own water-balance lookup tables (sourced from Taylor Technologies' Pool/Spa Water Chemistry reference; see the /reference/datasets/water-balance dataset page), not invented for this page. This site does not currently offer an interactive LSI calculator — this page explains the formula and its inputs only. See the Limitations section below.
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. This site's lookup tables only include values at fixed intervals (see /reference/datasets/water-balance); intermediate readings require interpolation, which this page does not attempt. No interactive LSI calculator exists on this site -- computing LSI currently requires manually reading all three factors from the published lookup tables, as shown above.
Last reviewed: 2026-08-28