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Calcium Saturation Index Calculator

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Calcium Saturation Index Formula

The following simplified (factor-table) formula is commonly used to calculate the Calcium Saturation Index (CSI) for pool/spa water. TF, CF, and AF are dimensionless factors obtained from standard tables (based on temperature, calcium hardness, and alkalinity). This form does not include an explicit total dissolved solids (TDS) term; other saturation-index formulations may add a separate TDS/ionic-strength correction.

CSI = pH + TF + CF + AF - 12.1

Variables:

  • CSI is the Calcium Saturation Index
  • pH is the measure of acidity or alkalinity of the water
  • TF is the Temperature Factor
  • CF is the Calcium Hardness Factor
  • AF is the Alkalinity Factor

To calculate the Calcium Saturation Index, add the pH, Temperature Factor, Calcium Hardness Factor, and Alkalinity Factor together. Then subtract 12.1 from the total.

What is a Calcium Saturation Index?

The Calcium Saturation Index (CSI) is a measure used in water chemistry to estimate whether water will tend to precipitate calcium carbonate (scale) or dissolve it (aggressive/etching water). In the common pool/spa factor-table approach, CSI is calculated from the water’s pH plus factors derived from temperature, calcium hardness, and alkalinity. A positive CSI indicates that the water is oversaturated with calcium carbonate and may cause scaling, while a negative CSI means the water is undersaturated and may tend to dissolve calcium carbonate (which can contribute to corrosion/etching issues).

How to Calculate Calcium Saturation Index?

The following steps outline how to calculate the Calcium Saturation Index (CSI).

  1. First, determine the pH value of the water.
  2. Next, determine the Temperature Factor (TF) from a standard CSI/LSI factor table for your water temperature.
  3. Next, determine the Calcium Hardness Factor (CF) from a standard factor table for calcium hardness (typically reported in ppm as CaCO3).
  4. Next, determine the Alkalinity Factor (AF) from a standard factor table for (carbonate) alkalinity (typically reported in ppm as CaCO3).
  5. Finally, calculate the Calcium Saturation Index (CSI) using the formula: CSI = pH + TF + CF + AF – 12.1.

Example Problem:

Use the following variables as an example problem to test your knowledge.

pH = 7.5

TF = 0.6

CF = 1.9

AF = 2.0

CSI = 7.5 + 0.6 + 1.9 + 2.0 – 12.1

CSI = -0.1