Pool Saturation Index Calculator
Enter pH, water temperature, calcium hardness, total alkalinity, and cyanuric acid to get your Langelier Saturation Index, and whether the water is corrosive, balanced, or scaling. It corrects alkalinity for CYA, so stabilized pools read accurately.
The water is balanced. It is neither dissolving surfaces nor depositing scale. Keep your levels where they are.
Balanced is roughly -0.3 to +0.3. Carbonate alkalinity used: 87 ppm (your total alkalinity corrected for cyanuric acid).
The Langelier Saturation Index combines pH, temperature, calcium hardness, and carbonate alkalinity into one balance number. It is the single best check that your water is neither eating your pool nor scaling it. Adjust one factor at a time and recheck.
The one number that ties it together
Each of your readings matters, but the saturation index is what tells you whether they add up to balanced water. If it is off, adjust the easiest factor: usually pH or alkalinity. Use the pH calculator, alkalinity calculator, or calcium hardness calculator to dial in whichever one moves it back toward zero.
Frequently asked questions
What is the pool saturation index?
The saturation index, or Langelier Saturation Index (LSI), is a single number that tells you whether your water is balanced, corrosive, or scaling. It combines pH, water temperature, calcium hardness, and carbonate alkalinity. A value from about minus 0.3 to plus 0.3 is balanced.
What does a negative saturation index mean?
Negative means the water is corrosive, or aggressive. It is hungry for calcium and will pull it from plaster, grout, and tile, and corrode metal parts and heaters. You fix it by raising pH, alkalinity, or calcium hardness.
What does a positive saturation index mean?
A value above plus 0.3 means the water is scaling. Calcium carbonate drops out of solution as cloudy water and hard scale on tile, surfaces, and especially the heater. You fix it by lowering pH or alkalinity.
Does cyanuric acid affect the saturation index?
Yes. Some of your total alkalinity reading is actually cyanurate, not carbonate, and only carbonate alkalinity belongs in the index. This calculator subtracts the cyanurate portion at your pH, so a stabilized pool reads correctly instead of looking more balanced than it is.
What is the ideal saturation index for a pool?
Aim for as close to zero as you can, comfortably within minus 0.3 to plus 0.3. Plaster and concrete pools care the most, since corrosive water etches the surface. Vinyl and fiberglass pools are more forgiving but still benefit from balanced water for equipment life.
Langelier Saturation Index using the standard industry factors (temperature factor, calcium factor as log of hardness, alkalinity factor as log of carbonate alkalinity), with carbonate alkalinity corrected for cyanurate at your pH. A constant of 12.1 is used, or 12.2 for high TDS saltwater pools.