Expert Verified Updated: March 2026

Calcium Saturation Index (CSI) Calculator

Calculate your exact CSI to determine if your pool water is aggressively dissolving your plaster (corrosive) or depositing scale (scaling). Trusted by TFP experts.

Quick Conclusion
The ideal Calcium Saturation Index (CSI) is exactly 0.0, but anything between -0.3 and +0.3 is perfectly balanced. A CSI below -0.6 will eat your plaster, while a CSI above +0.6 creates severe calcium scale.

Technical Comparison

CSI RangeWater ConditionAction Required
-0.6 or lowerHighly CorrosiveURGENT: Raise pH, Alk, or Calcium immediately
-0.3 to +0.3Ideal BalanceNone. Keep water here.
+0.6 or higherHighly ScalingURGENT: Lower pH with Muriatic Acid
Pool plaster is made of calcium. If your water's CSI is deeply negative, the water acts like a vampire, stealing calcium directly from your expensive plaster walls to satisfy its chemical hunger, ruining the finish.

Actionable Steps

1. Test your water for all 5 variables accurately using a drop-test kit. 2. Input the values into the CSI calculator. 3. Look at the result. If it's red/corrosive, add Baking Soda or Calcium. If it's scaling, add Muriatic Acid.

Pro-Tip

If you have a Salt Water Generator, intentionally keep your CSI slightly negative (around -0.2). Salt cells generate extreme heat and high pH internally, which creates scale. A slightly negative pool CSI prevents scale from building up inside the expensive cell plates.

Frequently Asked Questions

What is the difference between CSI and LSI?

CSI (Calcium Saturation Index) and LSI (Langelier Saturation Index) are very similar, but CSI adjusts Total Alkalinity by subtracting the Cyanuric Acid (CYA) buffering effect, making it much more accurate for outdoor residential pools.

How do I fix a negative CSI?

The easiest way to raise a negative CSI is to raise your pH slightly, or add Calcium Hardness Increaser (Calcium Chloride).

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