LSI (saturation index) explained
The Langelier Saturation Index tells you if water will etch plaster or leave scale. Formula, factor tables, and why temperature matters.
The LSI says whether water wants to dissolve calcium (negative, corrosive) or deposit it (positive, scale)[3].
LSI = pH + TF + CF + AF − C
- TF temperature factor: 32 °F 0.0 · 53 °F 0.3 · 66 °F 0.5 · 76 °F 0.6 · 84 °F 0.7 · 94 °F 0.8 · 105 °F 0.9[4]
- CF calcium factor = log₁₀(calcium hardness) − 0.4 (100 ppm → 1.6, 300 → 2.1, 400 → 2.2)[4]
- AF alkalinity factor = log₁₀(carbonate alkalinity), where carbonate alkalinity = TA − CYA/3[5]
- C = 12.1 for ordinary water; we use 12.2 for salt pools (high dissolved solids)[2]
Worked example
pH 7.6, 84 °F, calcium 300, TA 100, no CYA: 7.6 + 0.7 + 2.1 + 2.0 − 12.1 = +0.3 (balanced). The same water at 50 °F has a temperature factor of about 0.26 instead of 0.7, so it reads about −0.2: near the corrosive edge without any change in chemistry. That's why winter water needs a fresh LSI check.
What range to aim for
Taylor calls −0.3 to +0.5 balanced; many pros aim for −0.3 to +0.3[2]. We flag anything outside ±0.3.