Free tool
Langelier Saturation Index (LSI) Calculator
See your pool's full water balance at once — pH, temperature, calcium hardness, and alkalinity, combined into one corrosive-to-scale-forming score.
How to use this calculator
- 1Test your water
You'll need four readings: pH, water temperature, calcium hardness, and total alkalinity.
- 2Enter all four values
Total dissolved solids (TDS) is optional — the core LSI formula only needs the four values above.
- 3Read your LSI
The calculator computes LSI = pH + temperature factor + calcium hardness factor + alkalinity factor − 12.1 using standard published lookup tables.
- 4Check the classification
Balanced is between −0.3 and +0.3. Below that is corrosive; above it is scale-forming — adjust the driving factor (usually pH or alkalinity first) and retest.
Frequently asked
Questions about the LSI
What is the Langelier Saturation Index?
LSI is a single number, computed from pH, water temperature, calcium hardness, and total alkalinity, that estimates whether pool water will dissolve (corrode) or deposit (scale) calcium carbonate. It's the standard way to look at overall water balance rather than just one parameter at a time.
What does my LSI result mean?
An LSI between −0.3 and +0.3 is considered balanced. Below −0.3, water is corrosive and can etch plaster, grout, and metal equipment over time. Above +0.3, water is scale-forming and can deposit visible scale on surfaces, tile, and equipment. The further from zero, the more pronounced the effect.
How does this calculator handle the temperature, calcium, and alkalinity factor tables?
It uses the standard, widely-published LSI factor tables and linearly interpolates between the nearest published values rather than rounding to the closest one — this gives a smoother, more realistic number for readings that fall between table entries.
Does total dissolved solids (TDS) affect the result?
TDS is a secondary influence some published references incorporate into a more complex version of the formula. This calculator keeps the core four-factor formula (pH, temperature, calcium hardness, alkalinity) and treats TDS as optional advisory context — high TDS is flagged, but it doesn't change the numeric LSI shown here.