PoolDose Pool LSI calculator: is your water aggressive or scaling?
The Langelier Saturation Index adds up pH, temperature, calcium hardness and alkalinity (corrected for stabilizer) and subtracts a constant. Below -0.3 the water is aggressive, above +0.3 it is scaling, and in between it is balanced. The Water Index screen does it from five numbers off a strip.
What does the Langelier index tell you?
Aggressive water wants to dissolve things: plaster, grout, metal fittings. Scaling water drops calcium out as a crust on tiles and heaters. An index near zero means neither is pulling hard.
How does PoolDose calculate it?
- Index = pH + temperature factor + hardness factor + alkalinity factor - 12.1.
- The three factors are read from published tables and interpolated between the table points.
- Alkalinity is corrected first: stabilizer divided by three is subtracted before the lookup, with a floor of 4.
- The constant is 12.1 for water under 1,000 ppm of dissolved solids, and the app always assumes 300 ppm. It never asks for that figure, so a salt pool with more dissolved solids is only approximated.
How do I use the Water Index screen?
- Open Calculator > Water Index.
- Under Your test strip, fill in pH, the temperature, Calcium hardness (ppm), Total alkalinity (ppm) and Stabilizer / CYA (ppm).
- Read the number under Langelier Saturation Index and the line below it: Water is aggressive, Water is balanced or Water is scaling.
- Read the small note under it. It names the first thing outside range: alkalinity outside 60-120 ppm, then pH outside 7.2-7.6, then calcium hardness outside 200-400 ppm.
| pH | Temp | Calcium hardness | Alkalinity | Index | Result and first fix |
|---|---|---|---|---|---|
| 7.4 | 25 °C | 250 | 80 | -0.32 | Aggressive, note says nothing obviously out of range |
| 8.0 | 30 °C | 400 | 150 | +0.88 | Scaling, fix alkalinity first |
| 7.0 | 20 °C | 150 | 60 | -1.18 | Aggressive, fix pH first |
| 7.4 | 28 °C | 300 | 100 | -0.07 (CYA 40) | Balanced |
The first row is the screen's own starting set, and it lands just under the -0.3 line. Raise the temperature to 80 °F (26.7 °C) and the same water reads -0.29, balanced. The index is a guide to a trend, not a verdict on a single reading.
Questions and answers
What is a good LSI for a pool?
In this app, anything from -0.3 to +0.3 counts as balanced. Below that it says aggressive, above it scaling.
Does the calculator need TDS?
No. It uses a fixed 300 ppm figure and never asks, so results for salt pools are approximate.
Why subtract stabilizer from alkalinity?
Cyanuric acid shows up on an alkalinity test without adding real buffering. The app takes a third of the CYA reading off before looking up the factor.
Can the index be negative with everything in range?
Yes, the first example above does exactly that. In-range readings can still sum to slightly aggressive water, and the screen will say so.