Pool LSI Calculator

Calculate the Langelier Saturation Index (LSI) for pool water using the Wojtowicz CSI formula with Henderson-Hasselbalch and Debye-Hückel corrections. Supports cyanuric acid, borates, and salt inputs. Determine if your water is balanced, corrosive, or scale-forming.

Read more about the Langelier Saturation Index on Wikipedia
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-0.5 -0.3 0 0.3 0.5
Corrosive Slightly Soft Balanced Slightly Hard Scaling

More about the Pool LSI Calculator

This calculator uses the Wojtowicz Calcium Saturation Index (CSI) formula, the most thermodynamically accurate method for calculating pool water balance. The base formula comes from Wojtowicz (2001) "Factors Affecting the Calcium Carbonate Saturation Index", enhanced with Henderson-Hasselbalch CYA/borate corrections and Debye-Hückel activity coefficients.

Formula
CSI = pH + log10(CH) + log10(TA − 0.38772 × CYA1 + 10(6.83 − pH) − 4.63 × Borates1 + 10(9.11 − pH)) − 2.56√(1.5 × CH + TA50045 + max(0, Salt − 1.1678 × CH)58440)1 + 1.65√(1.5 × CH + TA50045 + max(0, Salt − 1.1678 × CH)58440) − 1412.5Temp + 273.15 − 6.9395
Where
  • Temp = temperature in °C
  • CH = calcium hardness (ppm)
  • TA = total alkalinity (ppm)
  • CYA = cyanuric acid / stabilizer (ppm)
  • 0.38772 = CaCO₃ equivalent weight / cyanuric acid molecular weight
  • 6.83 = pKa of cyanuric acid (first ionization, pool conditions)
  • 4.63 = CaCO₃ equivalent weight / boron atomic weight
  • 9.11 = pKa of boric acid (pool conditions)
  • 2.56 = Debye-Hückel charge factor (z² sum of Ca²⁺ and HCO₃⁻)
  • 1.65 = Debye-Hückel ion size factor
  • 50045 = ionic strength divisor for calcium and carbonate ions (from CaCO₃ molecular weight)
  • 58440 = ionic strength divisor for sodium chloride (NaCl molecular weight × 1000)
  • 1412.5 = temperature correction constant for CaCO₃ solubility
  • 273.15 = Celsius to Kelvin conversion offset
  • 6.9395 = combined equilibrium constant for calcium carbonate
Interpretation
  • LSI < −0.5: Corrosive / leeching: water will dissolve pool surfaces
  • LSI −0.5 to −0.3: Slightly soft: mildly corrosive
  • LSI −0.3 to +0.3: Balanced: ideal range
  • LSI +0.3 to +0.5: Slightly hard: mild scaling risk
  • LSI > +0.5: Scaling: calcium carbonate will precipitate