Water balance by LSI
One number that says whether the water attacks the pool or coats it.
Before you start
Chlorine and pH being right does not mean the water is right. The Langelier Saturation Index combines temperature, pH, alkalinity, calcium hardness and dissolved solids into one figure.
Step by step
- Open Pool calculator.
- Enter temperature, pH, alkalinity and calcium hardness. All four are needed — without any one of them there is no LSI.
- Read the value and the verdict.
How to read it
| Value | Meaning |
|---|---|
| −0.3 to +0.3 | balanced |
| above +0.3 | scale-forming — deposits on tiles, heater and salt cell |
| below −0.3 | corrosive — attacks grout, plaster and metal parts |
Between 0.1 and 0.3 either way it says slightly and asks you to watch it. The colour follows the same three steps.
Good to know
This is why calcium hardness and temperature are worth measuring, even though neither is something you dose weekly. Water that is perfectly chlorinated and out of balance still eats the grout.
Warm water shifts the index upwards. The same water that is balanced in March is scale-forming in an unshaded pool in August — which is exactly when nobody measures it.
What it does not do
It does not tell you what to change. It says which way the water is out; which of the four values you move is a decision that depends on the pool.
Without a temperature reading there is no LSI at all, and the field stays empty rather than assuming 25 °C.
Last updated 2026-08-21