The Langelier Saturation Index (LSI) tells you whether water will dissolve surfaces, do nothing, or drop scale. Zero is balanced, and about -0.3 to +0.3 is the working band. A pool at pH 7.4, alkalinity 80 ppm, calcium 200 ppm, 82 °F and CYA 40 ppm scores -0.40: slightly corrosive, even though every number is inside its normal range.
How the LSI is calculated
LSI = pH − pHs, pHs = 9.3 + A + B − (C + D)
A is (log10 of dissolved solids − 1) ÷ 10. B is −13.12 × log10(°C + 273) + 34.55, the temperature term. C is log10 of calcium hardness minus 0.4. D is log10 of carbonate alkalinity. The formula is Langelier's, as on Wikipedia. The one pool-specific step is D: stabilizer (cyanuric acid) counts in the total alkalinity test but does nothing for scale, so we use carbonate alkalinity, which is total alkalinity minus CYA times a factor that grows from 0.23 at pH 7.0 to 0.36 at pH 8.0 (PHTA alkalinity fact sheet: it should always be used for LSI). In the example: 80 − 40 × 0.31 = 68 ppm carbonate alkalinity, pHs = 9.3 + 0.20 + 2.03 − (1.90 + 1.83) ≈ 7.80, so LSI = 7.4 − 7.80 = -0.40.
Two formulations, two answers
You will meet a formula with logarithms, which we use, and a chart method with factor tables for temperature, calcium and alkalinity, ending in a constant of 12.1. The chart rounds each factor, so the two differ by up to about 0.15. For pH 7.6, 84 °F, calcium 300 ppm, alkalinity 90 ppm and 1,000 ppm dissolved solids, the chart method gives +0.25 and the formula +0.12 with no CYA, and +0.05 with CYA 40 ppm. A third version, the calcite saturation index used by Trouble Free Pool, also counts ionic strength and salt; we show it under the result (-0.30 for the default water). PHTA's recommended range is -0.3 to +0.5; Orenda and Trouble Free Pool use -0.3 to +0.3, and so do we, because it gives scale less room. Say which method you used when you compare numbers.
What to change first
Below the gauge, the table shows the one change that would make LSI zero on its own for pH, alkalinity and calcium, and whether it falls in the acceptable range (pH 7.2 to 7.8, alkalinity 60 to 180 ppm, calcium 150 to 2,500 ppm). For the default water pH would need 7.82 and alkalinity 184 ppm, both outside their ranges, so calcium hardness at about 506 ppm is the single fix. Usually you move two small things instead: a little more alkalinity and a little more calcium. Scaling water is the mirror case: lower pH first, because acid is quick, and lower alkalinity if pH keeps rebounding. Amounts for either are in the pool chemical calculator; calcium alone is in the pool calcium hardness calculator.
Temperature, TDS and salt
Warm water holds less dissolved calcium, so LSI rises about 0.2 for every 10 °C. The table under the gauge shows your water at 50, 68, 86 and 100 °F, so a balanced summer pool can be corrosive by winter. Leave total dissolved solids blank and the calculator uses 1,000 ppm plus your salt. The effect is small: 1,000 to 4,600 ppm changes LSI by about 0.07.
Pool and spa
Choose Spa or hot tub for hot-tub temperatures (default 102 °F) and no CYA. Spa water above 104 °F (40 °C) is unsafe under the CDC Model Aquatic Health Code, and hot water scales fast, so watch calcium. Spa dosing has its own page: hot tub chemical calculator.
Reading the numbers with plaster, vinyl and fiberglass
Corrosive water matters most for plaster, grout and heat exchangers, because they contain or sit next to calcium and metal. Vinyl and fiberglass do not dissolve, but scale and staining still form on them. If you have plaster, set calcium from your surface and work toward LSI near zero. See every tool in pool chemistry; if pH is the culprit use the pool pH calculator, and if alkalinity is, the pool alkalinity calculator. Your gallons are needed for doses: pool volume calculator.