Pool Saturation Index Calculator

What Is the Langelier Saturation Index (LSI)?

The Langelier Saturation Index, or LSI, is a scientific way to determine whether water will dissolve or deposit calcium carbonate. It tells you how “aggressive” or “scale-forming” your pool water is.

• A negative LSI (< -0.3) means your water is corrosive. It can dissolve calcium from plaster, metal fittings, and grout.
• A positive LSI (> +0.3) means your water is scaling. It can leave white deposits on pool walls, filters, and heating elements.
• A balanced LSI (-0.3 to +0.3) means your water chemistry is perfect — not too acidic, not too alkaline.

The goal is to keep your LSI as close to zero as possible, ensuring your water is stable and comfortable for swimmers.

How the Pool Saturation Index Calculator Works

The calculator uses a simplified version of the LSI formula to give fast, reliable results:

LSI = pH + TF + CF + AF – 12.1

Here’s what each factor means:

• pH: Measures how acidic or basic your pool water is.
• TF (Temperature Factor): Based on your pool water temperature (°F). Warm water tends to cause scaling faster than cold water.
• CF (Calcium Factor): Depends on your calcium hardness level.
• AF (Alkalinity Factor): Depends on total alkalinity, which helps buffer pH changes.
• 12.1: A constant that represents total dissolved solids (TDS) and other water balance factors.

The calculator automatically applies these factors and shows your LSI value and water status — color-coded for easy understanding.

How to Use the Pool Saturation Index Calculator

1. Enter Your Pool’s pH
   Input your current pH reading (usually between 6.5 and 8.5).
2. Add Water Temperature
   Enter your pool’s water temperature in Fahrenheit.
3. Enter Total Alkalinity (ppm)
   Add your test result for alkalinity, which stabilizes your pool’s pH.
4. Enter Calcium Hardness (ppm)
   Input your calcium hardness level — too little causes corrosion, too much causes scale.
5. Click “Calculate Saturation Index”
   The calculator instantly displays your LSI value and shows whether your water is balanced, corrosive, or scaling.

If needed, you can click “Reset” to start fresh with new values.

Understanding Your LSI Results

1. Corrosive (LSI < -0.3)
Water is too aggressive. It’s likely to etch plaster, corrode metal, and damage pool finishes. To correct this:

• Raise calcium hardness or alkalinity
• Adjust pH upward using a pH increaser

2. Balanced (-0.3 ≤ LSI ≤ +0.3)
Congratulations — your water chemistry is stable! You’re protecting your pool and ensuring a smooth swimming experience.

3. Scaling (LSI > +0.3)
Your water is over-saturated with calcium, which can cause scale buildup on tiles, filters, and heaters. To correct this:

• Lower pH slightly using muriatic acid
• Reduce alkalinity if it’s too high

Why LSI Balance Matters

Maintaining the right Saturation Index helps you:

• Protect pool surfaces from corrosion or rough deposits
• Extend equipment life by preventing damage to pumps and heaters
• Save on maintenance costs by reducing chemical and cleaning needs
• Improve swimmer comfort with water that feels softer and cleaner

A stable LSI is the foundation of efficient, low-maintenance pool care. It’s like finding your pool’s “sweet spot” where everything works in harmony.

Example Calculation

Let’s say your pool has the following readings:

• pH = 7.5
• Temperature = 80°F
• Total Alkalinity = 100 ppm
• Calcium Hardness = 250 ppm

Using the formula, your calculator gives an LSI close to 0.0, meaning your pool water is balanced and stable — perfect for daily use.

If the LSI had dropped to -0.5, it would mean the water is slightly corrosive, signaling you should increase alkalinity or calcium hardness to restore balance.

Smart Pool Maintenance Starts Here

The Pool Saturation Index Calculator is more than just a water testing tool. It’s a practical guide to help you understand and maintain pool chemistry without advanced chemistry knowledge. By using it regularly, you can catch imbalances early, prevent costly repairs, and keep your pool sparkling clean.

Remember: a perfectly balanced pool doesn’t just look better — it lasts longer and feels better too.

Disclaimer

This calculator follows the standard Langelier formula for approximate results. Factors such as TDS, cyanuric acid, and local water chemistry can slightly affect accuracy. Always confirm results with a professional water test if your pool shows persistent imbalance. Keep your LSI between -0.3 and +0.3 for optimal water balance.