The Langelier Saturation Index (LSI) is the single number that predicts whether your water will scale (deposit calcium) or etch (dissolve it out of surfaces). Every experienced pool operator watches LSI more than any individual parameter.
Target: -0.3 to +0.3. Outside that window, water becomes aggressive (etching) or scaling.
The formula
LSI = pH + TF + CF + AF − TDSF
- pH — your measured pH
- TF — temperature factor
- CF — calcium hardness factor
- AF — carbonate alkalinity factor (TA minus CYA correction)
- TDSF — total dissolved solids factor
You don't need to do this by hand. Use a pool calculator (or our team's app) — just plug in your test numbers and it gives you the index.
Why LSI matters in Florida
High summer temperatures push LSI positive (scaling). Hard fill water pushes it even higher. Many Florida pools run LSI of +0.6 in August without the owner realizing — and they end up with scaled salt cells and milky water that won't clear.
How to correct LSI
Fix from the biggest lever first:
- If pH is high, lower it (biggest single lever).
- If TA is very high, lower it toward 80 ppm.
- If CH is above 400 ppm, plan a dilution.
Temperature is out of your control — but it's exactly why summer chemistry in Florida is harder than winter chemistry.