Pool chemical calculator with LSI.
Enter one water test. Get your Langelier Saturation Index and exact doses to hit target, in the order you should add them.
Doses to target
Targets: FC 3 · pH 7.5 · TA 90 · CH 300 · CYA 40 · Salt 3200. Rule-of-thumb doses for standard product strengths. Add part, circulate, retest. The label on the bucket wins.
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How to read your LSI
The Langelier Saturation Index tells you whether water will dissolve things (corrosive) or deposit things (scale). It combines pH, temperature, calcium hardness and carbonate alkalinity into one number.
| LSI | What it means | What you'll see |
|---|---|---|
| Below −0.3 | Corrosive | Etched plaster, pitted metal, heater damage |
| −0.3 to −0.2 | Slightly corrosive | Slow wear over the season |
| −0.2 to +0.2 | Balanced | Where you want to live |
| +0.2 to +0.3 | Slightly scale-forming | Early scale at the waterline |
| Above +0.3 | Scale-forming | Cloudy water, scale on tile, clogged salt cells |
The formula
LSI = pH + TF + (log₁₀ CH − 0.4) + log₁₀(carbonate alkalinity) − C. TF is the temperature factor. Carbonate alkalinity is TA minus about a third of your CYA. C is 12.1, or 12.2 for salt pools, where dissolved solids run high.
Common questions
What is a good LSI for a pool?
Between −0.3 and +0.3 is balanced. Closer to 0 is better. Below −0.3 the water is corrosive and can etch plaster and damage metal. Above +0.3 it is scale-forming.
Why does cyanuric acid change the LSI?
Part of total alkalinity is cyanurate, not carbonate. The LSI uses carbonate alkalinity, so this calculator subtracts about one third of your CYA from TA before calculating.
In what order should I add pool chemicals?
Adjust alkalinity first, then pH, then calcium, then stabilizer, and sanitizer last. Add one chemical at a time, circulate, and retest before the next.
How do I lower CYA?
Only by dilution. Partially drain and refill. The calculator estimates how much water to replace.