About this tool
The LSI Water Balance Calculator finds the Langelier Saturation Index of your pool water — the professional standard for telling whether the water is corrosive (aggressive), balanced, or scale-forming. Balanced water protects plaster, grout, metal, and equipment; water that drifts low etches and dissolves surfaces, while water that drifts high deposits calcium scale. Everything is calculated locally in your browser, so nothing you enter is uploaded.
The index is LSI = pH + TF + CF + AF − constant. The temperature factor TF is read from the standard table (0.0 at 32°F rising to about 0.9 at 105°F, interpolated). The calcium factor is CF = log₁₀(calcium hardness) − 0.4. Because cyanuric acid contributes to your total alkalinity titration, the tool first corrects it: carbonate alkalinity = total alkalinity − CYA × f, where f depends on pH (about 0.19 at pH 7.0 up to 0.36 at pH 8.0); then AF = log₁₀(carbonate alkalinity). The constant is 12.1 for TDS under 1,000 ppm and 12.2 above it.
The verdict follows the PHTA/ANSI-APSP-11 balance band: an LSI below −0.3 is corrosive, between −0.3 and +0.3 is balanced, and above +0.3 is scale-forming. Enter pH, water temperature (°F or °C), calcium hardness, total alkalinity, cyanuric acid, and TDS, and the calculator shows the LSI, the verdict, and each factor (TF, CF, AF) plus the cyanurate-corrected carbonate alkalinity so you can see what to adjust. Raising pH, alkalinity, calcium, or temperature increases LSI; lowering them decreases it.