Langelier Saturation Index (LSI): understanding your pool water balance

What is the Langelier Saturation Index?

The Langelier Saturation Index (LSI) measures water's tendency to dissolve or deposit calcium carbonate. It compares the actual pH of the water with the pH at which it would be exactly saturated with calcium carbonate (the saturation pH, or pHs): LSI = pH − pHs.

In practice, water that is short of calcium carbonate takes it from elsewhere: grout, plaster, metal parts. Water with too much deposits it on walls, in the filter and in the heater.

Readings used in the calculation

• pH: the factor with the biggest influence on the index.
• Water temperature: with identical readings, warm water is more scaling than cold water.
• Calcium hardness: the amount of dissolved calcium.
• Total alkalinity: the buffer that keeps pH stable. If you use stabilizer (cyanuric acid), part of the measured alkalinity comes from it; accurate calculations correct for this.
• Total dissolved solids (TDS): a smaller effect, but still included, especially for salt pools.

How to read the result

LSI value	Water condition	Risk
Below −0.3	Corrosive (aggressive)	Etched grout and plaster, corroded metal, rough surfaces
−0.3 to +0.3	Balanced	Target range
Above +0.3	Scaling	White deposits, cloudy water, clogged filter and heater

These thresholds are the most widely used reference points. Your surface or equipment manufacturer's recommendations always come first.

Rebalancing corrosive or scaling water

Start with the reading furthest from its usual range, then test again:

• Corrosive water (LSI too low): usually raise alkalinity first, then pH, and calcium hardness if needed.
• Scaling water (LSI too high): usually lower pH; if calcium hardness is very high, partially replacing the water is often the only lasting fix.

Make small corrections with the pump running and retest after a few hours. One large correction often swings the water from one extreme to the other. Always follow the safety and dosage instructions on the products.

How often should you test?

During the season, pH and sanitizer are usually checked several times a week; alkalinity and calcium hardness change more slowly and are typically tested once or twice a month. Recalculate the index after any notable change: refilling, heavy rain, a heat wave or a shock treatment.

Tracking your LSI over time

A single number says little; the trend shows whether your water is drifting. Keeping a history of your tests lets you spot alkalinity falling week after week, or hardness climbing as you top up the pool.