What is LSI and why does your pool need it?
Most pool owners track chlorine and pH. Some check alkalinity. But very few know about the one number that tells you whether your water is slowly destroying your pool — or building up scale that clogs your equipment.
That number is the Langelier Saturation Index, or LSI.
The short version
LSI is a single number that tells you whether your pool water is balanced, corrosive, or scale-forming. It takes into account five factors that interact with each other — pH, temperature, alkalinity, calcium hardness, and TDS — and boils them down to one score.
| LSI Value | What It Means | What Happens |
|---|---|---|
| Below −0.3 | Corrosive | Water dissolves plaster, etches surfaces, corrodes metal fittings and heater cores |
| −0.3 to −0.1 | Slightly corrosive | Slow surface damage over time — hard to notice until it's expensive |
| −0.1 to +0.1 | Balanced | Water is satisfied — not attacking surfaces, not depositing scale |
| +0.1 to +0.3 | Slightly scale-forming | Mineral deposits start building up in pipes and on surfaces |
| Above +0.3 | Scale-forming | Calcium deposits clog heaters, SWG cells, and plumbing — cloudy water |
The ideal range varies slightly depending on your climate — pools in hot, arid regions like Arizona or inland California can tolerate slightly higher LSI because evaporation concentrates minerals faster. But the principle is the same everywhere: you want your water to be satisfied, neither hungry for minerals nor overloaded with them.
Why pH and chlorine aren't enough
Here's the problem most pool owners run into: every individual reading looks fine, but the water is still causing damage.
Your pH might be 7.5 (perfect). Your alkalinity might be 90 ppm (fine). Your calcium might be 250 ppm (looks good). But depending on your water temperature and how those numbers interact, your LSI could be −0.4 — meaning your water is actively corroding your pool surfaces.
You'd never know by looking at the individual numbers. LSI is the only way to see the full picture.
Real-world example
A pool with pH 7.4, alkalinity 80, calcium 200, and water temp of 60°F has an LSI of about −0.5. That's corrosive enough to etch plaster and pit metal fixtures — even though every individual reading looks "in range."
How LSI is calculated
The LSI formula takes your water's actual pH and combines it with factor values derived from your temperature, calcium hardness, alkalinity, and TDS:
Each factor is calculated from your readings — temperature uses a curve fitted to industry reference tables, calcium and alkalinity use logarithmic conversions, and TDS accounts for dissolved solids (relevant for salt pools).
One important detail that many basic LSI calculators skip: cyanuric acid (CYA) affects alkalinity's contribution to LSI. CYA acts as a buffer that absorbs some of the alkalinity's influence on water balance. A proper LSI calculation uses corrected alkalinity — your total alkalinity minus one-third of your CYA level — rather than raw TA. Without this correction, your LSI reading will be off, sometimes significantly.
You don't need to memorize any of this. The important thing is understanding that LSI connects five variables that individually might look fine but together can tell a very different story.
The five factors that affect LSI
- pH — The biggest lever. Raising pH pushes LSI up (toward scale), lowering it pushes LSI down (toward corrosion).
- Water temperature — Warmer water has a higher LSI. This is why pools in summer can tip toward scale-forming even if chemistry hasn't changed.
- Calcium hardness — More calcium means higher LSI. Too low and your water becomes hungry for calcium — and will pull it from your plaster.
- Total alkalinity — Acts as a pH buffer and influences LSI. For pools with CYA (stabilizer), the corrected alkalinity is used: TA minus one-third of CYA.
- TDS (Total Dissolved Solids) — Minor effect, but high TDS in saltwater pools can shift LSI slightly.
What to do if your LSI is off
LSI too low (corrosive):
- Raise calcium hardness with calcium chloride
- Raise alkalinity with sodium bicarbonate (baking soda)
- Raise pH with soda ash (sodium carbonate) — this also raises alkalinity slightly
LSI too high (scale-forming):
- Lower pH with muriatic acid
- Lower alkalinity with muriatic acid (add slowly, aerate to let pH recover)
- If calcium is very high, partial drain and refill with fresh water
Tip: adjust one thing at a time
Because LSI factors interact, changing one can shift others. Make one adjustment, wait for it to circulate (at least a few hours), retest, then decide if you need another change.
How often should you check LSI?
Every time you test your water. If you're already logging pH, alkalinity, calcium, and temperature, calculating LSI takes no extra effort — it's just math applied to numbers you already have.
Pay special attention to LSI after these events:
- Heavy rain (dilutes calcium and alkalinity, drops LSI)
- Temperature swings (seasonal changes shift LSI significantly)
- Adding chemicals (especially acid or calcium)
- After a partial drain and refill
The bottom line
Individual pool chemistry readings tell you part of the story. LSI tells you the whole story. A pool with "good" numbers can still be corroding or scaling — and the damage adds up silently until it becomes an expensive repair.
If you're only checking chlorine and pH, you're flying blind on water balance. LSI is the missing piece.
Track your LSI automatically
PoolChem Tracker uses the full LSI formula — including CYA-corrected alkalinity, polynomial temperature factors, and salt-adjusted TDS — not the simplified version most calculators use. Log your readings and get an accurate LSI with correction advice, every time.
