Pool LSI calculator
The Langelier Saturation Index tells you whether pool water will eat at plaster and metal, or leave scale behind. Enter a water test and get the answer in plain words.
The water is neither eating the surface nor leaving scale. Nothing to change.
What to do: Keep testing weekly; pH and temperature changes move the number.
Balanced is -0.3 to +0.3. Carbonate alkalinity used: 74 ppm (total alkalinity minus the part that comes from CYA).
How it works
LSI = pH - pHs, where pHs is the pH at which the water would be exactly saturated with calcium carbonate. pHs comes from temperature, calcium hardness, carbonate alkalinity and total dissolved solids.
Below -0.3 the water is corrosive; above +0.3 it is scaling; in between it is balanced. pH moves the LSI the most, which is why a small acid dose often fixes a scaling reading.
Questions
What is a good LSI for a pool?
Anywhere from -0.3 to +0.3 is balanced. LSI rises as water warms, so a pool that is balanced in winter can read scaling in summer.
Why does CYA change the LSI?
Cyanuric acid shows up in the total alkalinity test even though it doesn't buffer like carbonate does. The calculator subtracts that part (about a third of the CYA, depending on pH) to get carbonate alkalinity, which is what the LSI needs.
Does temperature matter?
Yes. Warm water holds less calcium, so the same water reads more scaling in August than in January. Use the water temperature on the day you test.
What is TDS and why is salt asked?
Total dissolved solids nudge the LSI slightly. Without a TDS reading the calculator assumes 1,000 ppm; for a salt pool it uses your salt level plus 300.
Run this math on every stop.
ShockRoute does the dosing for each pool's gallons in the tech app, logs it, and prices it from your supply costs. Free up to 25 pools.