Langelier Saturation Index (LSI)
The Langelier Saturation Index (LSI) is a single number that tells you whether your pool water will dissolve surfaces (corrode), form mineral scale, or stay balanced. It combines pH, temperature, calcium hardness, alkalinity, and TDS into one value. Target: -0.3 to +0.3, with zero being perfectly balanced.
The index was introduced by Dr. Wilfred F. Langelier in a 1936 paper published in the Journal of the American Water Works Association (Vol. 28, No. 10), where it was developed as a tool for controlling anti-corrosion water treatment.
Target ranges
| LSI Value | What it means | What happens |
|---|---|---|
| < -0.5 | Very corrosive | Aggressively dissolves plaster, etches grout, attacks metal fittings |
| -0.5 to -0.3 | Corrosive | Slow damage to surfaces over weeks/months |
| -0.3 to +0.3 | Balanced | Ideal — water neither dissolves nor deposits minerals |
| +0.3 to +0.5 | Scaling | Calcium starts precipitating; mild cloudiness possible |
| > +0.5 | Heavy scale | Visible scale on tile, heater coils, SWG cells; cloudy water |
A note on the target range: The Pool & Hot Tub Alliance (PHTA), the industry standards body, sets the acceptable LSI range at −0.3 to +0.5 under ANSI/APSP/ICC-11. PoolChem Tracker targets a more conservative −0.3 to +0.3. In hot, arid climates, where high water temperatures naturally push the LSI positive and hard fill water steadily raises calcium, that extra margin matters. Water at +0.4 meets the industry standard but will still leave deposits on tile lines, heater coils, and salt cells over time.
What LSI does
Pool chemistry has five interlocking variables: pH, alkalinity, calcium hardness, temperature, and total dissolved solids. Any of them out of range can damage your pool — but they can also compensate for each other. Higher calcium can offset lower pH. Higher temperature accelerates scaling. The LSI calculates the net effect of all five at once and gives you one number to act on.
This matters because "my pH is in range" isn't enough to know your water is balanced. A pool at pH 7.5 with high TA, high CH, and 90°F water is scaling hard. A pool at pH 7.5 with low CH and cool water is corrosive. Same pH, opposite problems.
The formula
Where:
- pH — your current pH reading
- TF — temperature factor (warmer water = higher TF = higher LSI)
- TDS — total dissolved solids factor (from salt level; small but measurable)
- CF — calcium hardness factor
- AF — CYA-corrected alkalinity factor (this is where most calculators get it wrong)
- 12.1 — fixed constant
Why CYA-corrected alkalinity matters
Total alkalinity has two parts: the "real" carbonate buffer that controls pH stability, and the "cyanurate" buffer from any CYA you've added. The cyanurate portion shows up on your TA test but doesn't actually buffer pH the same way. Using raw TA in the LSI formula overstates your alkalinity — sometimes by a lot.
The correction: subtract CYA / 3 from your tested TA before plugging into LSI. So if your TA reads 100 and your CYA is 60, your effective TA for LSI is 100 - 20 = 80.
This is why most LSI calculators are wrong
Most online calculators and pool store charts use raw TA. They tell you your water is "balanced" or "scaling" when it isn't. Apps that skip CYA correction give consistently wrong answers — especially for SWG owners running CYA 60-80. PoolChem Tracker uses the corrected formula by default. See how it's calculated.
How to interpret LSI
- Negative LSI = aggressive / corrosive water. Your water "wants" more dissolved minerals and will eat surfaces to get them.
- Zero LSI = saturated. Water is in chemical equilibrium with its surroundings.
- Positive LSI = supersaturated. Excess minerals come out of solution as scale.
A common misconception: negative LSI doesn't mean your water is "acidic." A pool at pH 7.6 can have negative LSI if calcium hardness is very low. LSI measures saturation, not acidity.
How to fix bad LSI
If LSI is too low (corrosive):
- Raise pH (most direct lever) — see baking soda vs soda ash
- Raise alkalinity if it's also low
- Raise calcium hardness if it's below 200 ppm
If LSI is too high (scaling):
- Lower pH (most direct lever) — see how to lower pH
- Lower alkalinity if elevated — see how to lower alkalinity
- Drain and refill if calcium hardness is above 500 ppm — no chemical removes calcium
Temperature affects LSI too, but you usually can't control pool temperature. Warm-pool owners run pH on the lower side of target to compensate.
Want to...
LSI calculated correctly, automatically
PoolChem Tracker uses the full Langelier formula with CYA-corrected alkalinity, polynomial temperature factors, and salt-adjusted TDS — not the simplified version most apps use.
