The index, unpacked
Water always wants a certain amount of dissolved calcium carbonate; how much depends on pH, temperature, alkalinity, and everything else dissolved in it. Undersaturated water takes calcium from wherever it can — plaster, grout, metal heat exchangers. Oversaturated water gives it back as scale. The Langelier index measures the gap:
LSI = pH + temperature factor + calcium factor + alkalinity factor − TDS constant
The factor tables are the standard pool-industry ones (the same math behind dealer water-test printouts), with alkalinity corrected for stabilizer — CYA ties up part of what an alkalinity test reads, so a stabilized pool's effective alkalinity is lower than the strip suggests.
When to check it
Monthly is plenty for most pools, plus three moments that matter: before closing for winter (cold water turns the index sharply negative — close at high-normal pH), after a fresh fill (new water chemistry is a blank slate), and whenever you run a heater hard (scale forms first in the exchanger, where water is hottest). Plaster and tile pools care the most; vinyl and fiberglass mostly protect their metal parts.
Frequently asked questions
What is the Langelier Saturation Index?
A single number describing whether water wants to dissolve calcium carbonate (negative — corrosive) or deposit it (positive — scale-forming). It combines pH, temperature, calcium hardness, alkalinity, and dissolved solids. Between −0.3 and +0.3, water is satisfied and leaves your pool surfaces alone. It's the same index water utilities use for pipe protection, applied with the standard pool-industry factor tables.
My individual readings are all "in range" — can LSI still be off?
Yes, and that's exactly why it's worth checking. Low-normal pH + low-normal alkalinity + low-normal calcium in cold water stacks into genuinely corrosive territory, though every strip looked fine. The index catches the combination the individual ranges miss.
Why does temperature matter?
Calcium carbonate is less soluble in warm water, so the same chemistry that's balanced in July can turn corrosive in a 50°F pool. This is why winterized pools are usually closed with pH and alkalinity at the high end of normal — cold water eats plaster all winter otherwise, and why heaters scale first: the water at the heat exchanger is the hottest in the system.
What should I actually change if I'm out of balance?
pH is the biggest, fastest lever — a 0.2 move shifts the index by 0.2 directly. Alkalinity and calcium work too but move slower and cost more product. Corrosive? Nudge pH up toward 7.6. Scale-forming? Bring pH toward 7.4 and consider whether calcium or alkalinity is the structural culprit. The dosing calculator turns the decision into amounts.