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 − K
- pH — your measured pH
- TF — temperature factor
- CF — calcium hardness factor
- AF — carbonate alkalinity factor (TA minus CYA correction)
- K — TDS-dependent constant (~12.1 at TDS < 1,000 ppm; ~12.2 at TDS ≥ 1,000 ppm)
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.
Frequently asked questions
No. The formula is LSI = pH + TF + CF + AF minus K, but you do not need to run it yourself. Plug your test numbers into a pool calculator or our team's app and it hands you the index. Nobody on my routes does that arithmetic by hand.
K is the TDS-dependent constant in the equation. It runs around 12.1 when your total dissolved solids sit below 1,000 ppm, and around 12.2 once TDS climbs to 1,000 ppm or higher. It shifts with how loaded the water is, which is why higher-TDS pools land at a slightly different balance point.
High summer temperatures push LSI positive, and hard fill water pushes it higher still. I see plenty of pools running LSI around +0.6 in August without the owner ever knowing, and they end up with scaled salt cells and milky water that will not clear. Winter runs cooler, so the water sits calmer.
Go after the biggest lever first. If pH is high, drop it, that is your single biggest move. If total alkalinity is very high, bring it down toward 80 ppm. If calcium hardness is over 400 ppm, plan a dilution. Chase the pieces in that order instead of nudging everything at once.
Two directions, both bad. Push the index below minus 0.3 and the water turns aggressive, meaning it etches and dissolves calcium right out of your plaster and surfaces. Push it above plus 0.3 and the water scales, depositing calcium onto your finish and equipment. The safe lane is minus 0.3 to plus 0.3.
No, temperature is out of your hands. That is exactly why summer chemistry down here is harder than winter chemistry. Heat drives the index up on its own, so you manage the levers you can touch, pH, alkalinity, and calcium hardness, to hold the balance while the temperature works against you.
Want a pro to handle this?
Our CPO-certified techs run this exact playbook on every weekly service visit.