The Langelier Saturation Index combines four readings into a single corrosivity number. Here is the calculation in plain English.
The Langelier Saturation Index (LSI) tells you whether your water is going to deposit calcium scale or strip calcium off your equipment. A reading between -0.3 and +0.3 means your water is balanced. Outside that, it is either etching or scaling. The calculation takes a minute and uses readings you already have.
Four numbers: pH, total alkalinity, calcium hardness, and water temperature. You probably have all of these from a routine test.
A simple LSI calculator (free apps, websites, or a printable chart). The formal formula needs lookup tables for temperature and TDS factors, but a calculator removes that complexity.
LSI between -0.3 and +0.3: water is balanced. Calcium and the equipment are in equilibrium.
LSI below -0.3 (say -0.5): water is corrosive. It will strip calcium from grout, heaters, and pumps. The fix is usually higher pH, higher alkalinity, or higher calcium hardness.
LSI above +0.3 (say +0.6): water is scaling. Calcium will precipitate out as cloudy haze and scale on the waterline and heater. The fix is lower pH or careful dilution if calcium and alkalinity are also too high.
Every refill. Run the LSI on the new water before you do anything else. If your fill water is naturally outside the band, your dosing strategy must compensate from the start.
When you have an unexplained problem (etching, scale, persistent cloudiness). LSI is the diagnostic that often reveals the cause when individual readings look fine.
For a tub running on a high-end mineral cartridge or salt system. Both rely on LSI being right for the cell or mineral release to work properly.
Similar concept, slightly different formula. LSI is more widely used in the spa and pool industry. Ryznar is used more in industrial water treatment.
Most calculators take TDS as an input but use a default if you do not enter one. The TDS factor is small for typical hot tub ranges, so the default is usually accurate enough.