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LSI Calculator
Calculate Langelier Saturation Index (LSI) to check overall water balance.
Prevent corrosion and scaling by maintaining balanced water chemistry.
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💡 LSI combines pH, temperature, alkalinity, calcium, and TDS to predict water balance. Aim for -0.3 to +0.3.
LSI interpretation guide
| LSI Value | Water Condition | What It Means | Action Needed |
|---|---|---|---|
| Below -0.5 | Very corrosive | Aggressive water, rapid equipment damage | Urgent: Raise pH, alkalinity, or calcium immediately |
| -0.5 to -0.3 | Slightly corrosive | Water will slowly corrode metal and etch plaster | Raise pH, alkalinity, or calcium |
| -0.3 to +0.3 | ✅ Balanced | Ideal range - water is stable and non-corrosive | No action needed - maintain current levels |
| +0.3 to +0.5 | Slightly scale-forming | Water will slowly deposit calcium on surfaces | Lower pH or alkalinity |
| Above +0.5 | Very scale-forming | Rapid calcium scaling, cloudy water, equipment damage | Urgent: Lower pH, alkalinity, or calcium |
How LSI is calculated
The Langelier Saturation Index uses this formula:
LSI = pH - pHs
Where pHs (saturation pH) = (9.3 + A + B) - (C + D)
The factors:
- A = Temperature factor (0.0 at 32°F to 0.7 at 140°F)
- B = Calcium hardness factor (increases with calcium ppm)
- C = Total alkalinity factor (increases with alkalinity ppm)
- D = TDS factor (typically 0.1 for most pools at 1000 ppm)
What it means: LSI predicts whether calcium carbonate will precipitate out of solution (positive LSI = scaling) or dissolve from surfaces into solution (negative LSI = corrosive). Zero LSI means water is perfectly saturated with calcium and won't deposit or dissolve it.
How to fix LSI problems
To fix negative LSI (corrosive water)
Choose one or more of these adjustments:
- Raise pH with soda ash (fastest fix, raises LSI ~0.1 per 0.2 pH increase)
- Raise alkalinity with baking soda (also helps stabilize pH)
- Raise calcium hardness with calcium chloride (best for plaster pools)
Which to adjust? Start with whichever parameter is furthest below its ideal range. For example, if pH is 7.2 (low) and calcium is 150 ppm (low for plaster), raise both. If pH is 7.5 (perfect) but calcium is 100 ppm (very low), raise calcium only.
To fix positive LSI (scale-forming water)
Choose one or more of these adjustments:
- Lower pH with muriatic acid (fastest fix, lowers LSI ~0.1 per 0.2 pH decrease)
- Lower alkalinity with muriatic acid (also lowers pH)
- Lower calcium hardness by draining and refilling with soft water (last resort, slow and expensive)
Which to adjust? Usually lower pH first if it's above 7.6. If pH is already ideal, lower alkalinity. Lowering calcium requires draining and is only needed for severe cases (400+ ppm calcium).
Temperature effects
Higher water temperature increases LSI (more scale-forming). This is why heated pools and hot tubs tend toward scaling. In winter, cooler water lowers LSI (more corrosive). Adjust other parameters seasonally to compensate for temperature changes.
LSI calculator FAQs
What is the ideal LSI for a pool?
The ideal LSI is -0.3 to +0.3 (balanced water). LSI below -0.3 indicates corrosive water that will damage equipment, etch plaster, and corrode metal fixtures. LSI above +0.3 indicates scale-forming water that will create calcium deposits on tiles, inside pipes, and on heater elements. Zero LSI is perfectly balanced but hard to maintain — aim for -0.3 to +0.3.
What does LSI measure?
LSI (Langelier Saturation Index) measures overall water balance by combining pH, water temperature, total alkalinity, calcium hardness, and total dissolved solids (TDS). It predicts whether water will be corrosive (dissolving calcium from surfaces) or scale-forming (depositing calcium onto surfaces). LSI is calculated as: actual pH minus saturation pH.
How do I fix negative LSI?
To fix negative LSI (corrosive water): raise pH with soda ash, raise alkalinity with baking soda, or raise calcium hardness with calcium chloride. Start with the parameter furthest from its ideal range. Raising pH by 0.2 increases LSI by approximately 0.2. Retest LSI after each adjustment.
How do I fix positive LSI?
To fix positive LSI (scale-forming water): lower pH with muriatic acid (fastest), lower alkalinity with muriatic acid, or lower calcium by draining and refilling with soft water. Lowering pH is usually the easiest fix. Lowering pH by 0.2 decreases LSI by approximately 0.2. Avoid lowering pH below 7.2.
How often should I check LSI?
Check LSI monthly during swimming season, when making major chemical adjustments, when temperature changes significantly (seasonal), or if you notice scaling or corrosion signs. LSI is not a daily maintenance metric like pH and chlorine — use it to verify overall water balance and diagnose chronic problems like persistent scaling or equipment corrosion.
Why is my LSI positive even though my pH is perfect?
LSI combines multiple factors, not just pH. You can have perfect pH (7.4) but still have positive LSI if alkalinity or calcium is too high, or if water temperature is very warm. Check all parameters: pH, alkalinity (should be 80-120 ppm), calcium (should be 200-400 ppm for plaster, 150-250 for vinyl), and temperature. High temperature increases LSI even with ideal chemistry.
Does LSI matter for hot tubs?
Yes, but hot tub LSI is naturally higher due to temperature (100-104°F). Hot tub water at 104°F has LSI approximately 0.3-0.4 higher than the same chemistry at 80°F. To compensate, keep pH on the lower side (7.2-7.4), alkalinity moderate (80-100 ppm), and calcium moderate (150-200 ppm). Hot tubs are more prone to scaling due to heat.