Home Pool LSI Calculator
Pool LSI Calculator — Langelier Saturation Index
Estimate whether your water has a corrosive or scale-forming saturation tendency from pH, temperature, total alkalinity, calcium hardness, TDS and CYA.
💡 LSI at a glance
Common planning band: about −0.3 to +0.3
Negative LSI: greater corrosive/etching tendency
Positive LSI: greater calcium-scale tendency
Important: when CYA is above 30 ppm, this calculator applies Taylor's practical CYA ÷ 3 correction to measured total alkalinity.
💡 LSI is a balance indicator, not a guarantee that damage or scale will occur. Verify test readings before treating.
LSI interpretation guide
| Estimated LSI | Planning interpretation | Next step |
|---|---|---|
| Below −0.5 | Strong negative / corrosive tendency | Confirm readings; review pH, corrected TA and calcium together. |
| −0.5 to −0.3 | Mild negative tendency | Review the lowest balance factors before adjusting. |
| −0.3 to +0.3 | Common balanced planning band | Maintain and monitor. |
| +0.3 to +0.5 | Mild scale-forming tendency | Review pH, corrected TA, calcium and temperature. |
| Above +0.5 | Strong positive / scale-forming tendency | Confirm readings before corrective treatment. |
Taylor notes that industry teaching ranges vary; many pool/spa references now use a tighter band such as −0.3 to +0.3 for planning.
Fix your LSI — shop the right chemical:
- Soda ash (pH up) — raise LSI when water is corrosive
- Muriatic acid (pH down) — lower LSI when water is scale-forming
- Calcium hardness increaser — raise LSI for plaster and gunite pools
- Taylor K-2006 FAS-DPD test kit — measure all LSI parameters with lab accuracy
How LSI is calculated
Taylor publishes the relationship as saturation index = pH + temperature factor + log(calcium hardness) + log(corrected alkalinity) − a constant that accounts for ionic strength/conversions. The important detail is corrected alkalinity: measured TA includes a CYA contribution.
Taylor's current technical FAQ gives a practical correction when CYA is above 30 ppm: corrected alkalinity = measured TA − CYA ÷ 3. This calculator applies that rule before the alkalinity factor.
Borates and more detailed ionic corrections are not modelled here. For those systems, compare with a full professional water-balance calculator.
How to respond to an LSI result
LSI is driven by several interacting readings. A negative result does not mean “add calcium” automatically, and a positive result does not mean “drain” automatically. Confirm the tests and choose the adjustment that makes sense for your pool surface, equipment and chemistry.
- pH Calculator — model acid or soda-ash demand using current TA.
- Alkalinity Calculator — raise TA or stage an acid reduction.
- Calcium Hardness Calculator — raise calcium or assess dilution.
- Pool Dilution Calculator — model water replacement when high dissolved minerals require it.
Temperature, season and repeat LSI checks
Water temperature is part of the saturation-index calculation, so the same pH, alkalinity and calcium readings can produce a different LSI as the water warms or cools. Warmer water generally pushes calcium-carbonate saturation tendency upward; colder water generally pushes it downward.
Recalculate LSI after a meaningful pH, alkalinity or calcium adjustment, after a large refill or dilution, and when seasonal water temperature changes substantially. If one input looks surprising, confirm that test before treating the pool to an index number alone.
LSI is a balance tool, not a sanitizer test. A result near zero does not tell you whether chlorine/bromine, CYA, pathogens or every other water-quality factor is acceptable.
LSI calculator FAQs
What is a good LSI range for a pool?
A commonly used planning band is about −0.3 to +0.3. Taylor notes that wider ±0.5 teaching limits also exist, while the industry increasingly uses tighter ranges. Treat LSI as a tendency indicator and confirm the underlying test readings before adjusting.
Why does this LSI calculator ask for CYA?
CYA contributes to a total-alkalinity titration. Taylor's practical correction says that when CYA is above 30 ppm, subtract one-third of the CYA reading from measured TA before using alkalinity in the saturation-index calculation.
Does zero LSI mean the water is perfectly safe?
No. LSI addresses calcium-carbonate saturation tendency; it does not measure sanitizer adequacy, pathogens, metals, phosphates or every water-quality issue. Chlorine/bromine and pH still need to meet the operating guidance for the pool or spa.
What makes LSI go up?
Higher pH, higher corrected alkalinity, higher calcium hardness and higher water temperature generally push the saturation index upward. The factors interact, so adjust based on the actual chemistry rather than changing every parameter.
What makes LSI go down?
Lower pH, lower corrected alkalinity, lower calcium hardness and lower temperature generally push LSI downward. Confirm measurements before treating, especially if one input looks unusual.
Can I use measured total alkalinity without correcting for CYA?
If CYA is significant, doing so can overstate the alkalinity factor. This calculator applies Taylor's simple CYA correction above 30 ppm; more advanced systems may also need borate or other corrections.