What is the Langelier Saturation Index?
The Langelier Saturation Index is the standard pool water balance score. The Langelier Saturation Index is calculated from pH, temperature, calcium hardness, total alkalinity, and TDS. The index is also called LSI. Pool LSI is the single best summary of whether pool water is corrosive, balanced, or scaling. Pool LSI was published in 1936 by Wilfred Langelier.
The ideal LSI band is -0.3 to +0.3. According to the Pool & Hot Tub Alliance handbook, an LSI below -0.5 corrodes plaster and metal at measurable rates. Research from the National Plasterers Council shows that pools held within ±0.3 LSI for an entire season see no measurable etching or scaling.
How is LSI calculated?
The formula is LSI = pH + TF + CF + AF − TDSF. The factors come from standard lookup tables. The temperature factor runs 0.0 at 32°F to 0.9 at 104°F. The calcium factor runs 1.0 at 50 ppm to 2.6 at 800 ppm. The alkalinity factor runs 1.4 at 25 ppm to 2.5 at 800 ppm. The TDS factor is 12.1 below 1000 ppm and 12.2 above.
| LSI value | Status | Effect |
|---|---|---|
| < -0.5 | Corrosive | Plaster pits, metal rusts, grout dissolves |
| -0.5 to -0.3 | Slightly corrosive | Slow plaster etch |
| -0.3 to +0.3 | Balanced | No measurable damage |
| +0.3 to +0.5 | Slightly scaling | Cloudy water |
| > +0.5 | Scaling | Calcium scale on heaters and salt cells |
What pushes LSI off balance?
- Cold winter water drops LSI; pools drift corrosive.
- Hot summer water raises LSI; pools drift scaling.
- Low calcium hardness drops LSI sharply.
- High pH (above 7.8) raises LSI by 0.2–0.4 units.
- High TA (above 150 ppm) raises LSI by 0.1–0.2 units.
Why does temperature affect LSI?
Hot water dissolves less calcium. Pool water at 90°F holds calcium at the saturation edge sooner than the same pool at 70°F. The result is that pools scale in summer and corrode in winter at the same chemistry readings. Research from the Pool & Hot Tub Alliance shows that a 20°F temperature swing shifts LSI by 0.3 units. Adjust calcium and TA seasonally to track the band. Use the pool saturation index calculator to compute the value in seconds.
Is LSI the same as RSI?
No. The Ryznar Stability Index is a related but separate water-balance metric. RSI is more sensitive to temperature but harder to interpret in residential pools. The pool industry standard is LSI.