Free tool · LSI calculator
Is your water balanced, scaling or corrosive?
The Langelier Saturation Index (LSI) tells you whether pool water will etch plaster or lay down scale. Enter your readings. Between −0.3 and +0.3 is balanced.
CYA adds to the alkalinity reading, so we subtract a third of it.
Salt pools usually run 3,000 ppm or more.
Water is neither eating surfaces nor laying down scale. Keep it here.
Questions
What is a good LSI for a pool?
Between −0.3 and +0.3. Many service pros aim for slightly positive, around 0 to +0.2, to protect plaster.
How is LSI calculated?
LSI = pH + temperature factor + calcium factor + alkalinity factor − TDS factor. The alkalinity factor uses carbonate alkalinity, which is total alkalinity minus about a third of the CYA.
Why does temperature matter?
Warm water holds less calcium. The same pool can be balanced in April and scaling in August.
Does salt change LSI?
Yes, a little. Salt raises TDS, which slightly lowers LSI. Salt cells run hot, so scale forms there first.