LSI calculator
The Langelier Saturation Index tells you if your water is hungry for calcium (etching plaster) or ready to drop it (scaling). It combines pH, temperature, calcium, alkalinity and dissolved solids.
Your water is …
What each factor contributes
| Temperature factor | 2.03 |
| Calcium factor | 2.00 |
| Carbonate alkalinity (80 ppm) factor | 1.90 |
| TDS factor | 0.20 |
| Saturation pH (pHs) | 7.63 |
Five dials, one balance
Each input shifts the LSI by the logarithm of its value, which is why doubling calcium from 200 to 400 ppm only moves LSI by 0.3, while a pH change of 0.3 does the same thing instantly. Warm water pushes toward scale; cold toward etching.
+0.17
Balanced
These sliders drive the calculator above, so you can try a fix here before adding anything. Winter tip: an uncovered plaster pool at 50 °F with the same chemistry as summer is often 0.4 lower on the LSI, so raise pH or calcium slightly before closing.
The LSI formula
LSI = pH − pHs, with pHs = (9.3 + A + B) − (C + D), where A = (log₁₀ TDS − 1) ÷ 10, B = −13.12 × log₁₀(°C + 273) + 34.55, C = log₁₀(calcium hardness as CaCO₃) − 0.4, and D = log₁₀(carbonate alkalinity as CaCO₃). Carbonate alkalinity is total alkalinity minus the cyanurate share of your stabilizer (about a third of CYA at pH 7.5). The calculator computes that share exactly for your pH.
Questions pool owners ask
What is a good LSI for a pool?
Between −0.3 and +0.3 is considered balanced. Below −0.3 water tends to dissolve calcium from plaster and grout (etching) and attack metal; above +0.3 it tends to deposit scale on tile, in heaters and salt cells. Many plaster-pool pros aim for 0 to +0.3.
How is the Langelier Saturation Index calculated?
LSI = pH − pHs, where pHs is the pH at which water is exactly saturated with calcium carbonate. pHs = (9.3 + A + B) − (C + D), with A from total dissolved solids, B from temperature, C from calcium hardness and D from carbonate alkalinity (total alkalinity minus the cyanurate part).
What is the difference between LSI and CSI?
The Calcite Saturation Index (CSI) used in some pool courses is a refinement of the LSI that uses ionic strength instead of TDS and corrects alkalinity for borates and cyanurate. For normal pool water the two usually agree within about 0.1.
Does LSI matter for vinyl or fiberglass pools?
Less for the surface, but it still matters for heaters, salt cells and plumbing. Scale-forming water clogs heat exchangers and cells; corrosive water eats copper and metal parts. Fiberglass can also stain or scale with high LSI.
Why does the LSI go up in summer?
Warmer water holds less calcium carbonate in solution, so the same chemistry becomes more scale-forming. Hot spas and heated pools are the most prone to scaling; winter water is more aggressive.