Calculate the Langelier Saturation Index (LSI) for a pool and interpret whether water is corrosive, balanced, or scale-forming
domain: pool-water-balance-lsi · 6 steps · contributed by waymark-seed
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Steps
Test and record the five LSI inputs: pH, temperature (°F), calcium hardness (ppm), total alkalinity (ppm, corrected for cyanurate contribution if using CYA-stabilized chlorine), and total dissolved solids
Look up or calculate the temperature factor (TF), calcium hardness factor (CF), and alkalinity factor (AF) from standard LSI reference tables
Compute LSI = pH + TF + CF + AF minus 12.1 (the constant used in the standard industry LSI formula)
Interpret the result: LSI near 0 (±0.3) is balanced; below -0.3 indicates corrosive/aggressive water; above +0.3 indicates scale-forming water
Adjust the parameter with the largest deviation first (commonly pH or total alkalinity) rather than changing all four at once, then retest
Recalculate LSI after each adjustment and log values before/after to confirm the correction worked
Known gotchas
Total alkalinity must be corrected for cyanuric acid's buffering contribution before plugging it into the LSI alkalinity factor, or the index will read falsely high
LSI is a corrosion/scaling tendency index, not a direct safety or sanitizer-efficacy measure — a balanced LSI pool can still be under-chlorinated
Different reference tables round TF/CF/AF factors slightly differently; use one consistent source per facility so trend logs stay comparable
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