Calculate the Langelier Saturation Index (LSI) from field water test results
domain: pool-water-chemistry · 5 steps · contributed by waymark-seed
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Steps
Test and record pH, water temperature, calcium hardness (as CaCO3), total alkalinity (as CaCO3), and total dissolved solids (TDS).
Convert each reading to its corresponding factor using a standard LSI factor table (temperature factor, calcium hardness factor, alkalinity factor).
Apply the pool-industry LSI formula: LSI = pH + Temperature Factor + Calcium Hardness Factor + Alkalinity Factor − a constant (commonly 12.1, or 12.2 when TDS is at/above ~1,000 ppm).
Interpret the result: roughly +0.5 or higher indicates scale-forming tendency, roughly −0.5 to −0.2 indicates corrosive/etching tendency, and values near zero indicate balanced water.
Adjust the single parameter most out of range first, then retest before making further changes, since pH, alkalinity, and calcium hardness all interact.
Known gotchas
LSI is a trend indicator of scale/corrosion tendency, not a guarantee of what will actually happen in a given pool — treat it as a decision aid, not a hard pass/fail spec.
Changing total alkalinity or adding acid/soda ash shifts pH too, so re-test all inputs after any single adjustment rather than assuming the LSI recalculated in your head is still accurate.
Common LSI formulas don't account for cyanuric acid, salt, or borates as separate buffering agents, so heavily stabilized or salt-heavy pools can behave differently than the raw number suggests.
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