What the saturation index tells you
Water is always trying to reach a balance with calcium carbonate, the mineral in plaster, grout and scale. If it holds less than it could, it dissolves calcium carbonate from your pool’s surfaces. If it holds more, it drops the excess as scale. The Langelier Saturation Index (LSI) puts a number on which way it’s leaning:
- Below −0.3: corrosive. Expect etched plaster, rough surfaces and pitted metal over time.
- −0.3 to +0.3: balanced. Orenda, a pool chemistry company, uses this as the acceptable range, with 0.00 as perfect.
- Above +0.3: scale-forming. Expect white crust at the water line, cloudy water and scaled salt cells and heaters.
The formula
LSI = pH − pHs
pHs = (9.3 + A + B) − (C + D)
A = (log₁₀ TDS − 1) ÷ 10. B = −13.12 × log₁₀(temperature in kelvin) + 34.55. C = log₁₀(calcium hardness) − 0.4. D = log₁₀(carbonate alkalinity). pHs is the pH at which the water would be exactly saturated.Pool test kits measure total alkalinity, which includes cyanurate from stabilizer. Cyanurate doesn’t take part in forming scale, so we subtract it first: at pH 7.6 it’s about a third of the CYA reading, the rule Orenda and Taylor both use. We calculate the exact share for your pH.
Worked example
Orenda’s published example: pH 7.6, 84°F, CH 300, TA 90, CYA 60, TDS 1,000
- Carbonate alkalinity = 90 − 20 = 70 ppm
- A = 0.2, B = 2.012, C = 2.077, D = 1.845
- pHs = (9.3 + 0.2 + 2.012) − (2.077 + 1.845) = 7.589
- LSI = 7.6 − 7.589 = +0.01
Orenda’s table method gives 0.00 by hand and +0.02 in their app, so the formula and the tables agree.
Temperature matters more than people expect
Warm water holds less calcium carbonate, so the same water leans toward scale as it heats up and toward etching as it cools. That’s why heater exchangers scale first, and why a pool that’s balanced in summer can turn aggressive in winter.
The same water at different temperatures
Show the numbers
| Water °F | Chlorine pool LSI | Salt pool LSI |
|---|---|---|
| 50 | −0.42 | −0.48 |
| 60 | −0.3 | −0.37 |
| 70 | −0.2 | −0.26 |
| 80 | −0.09 | −0.15 |
| 90 | +0.02 | −0.05 |
| 100 | +0.12 | +0.06 |
How to move the index
pH is the quickest lever. Raising pH by 0.1 raises the LSI by 0.1. Alkalinity and calcium are slower but longer lasting.
| If the LSI is | Try first | Then |
|---|---|---|
| Too low (corrosive) | Raise pH toward 7.6 to 7.8 with the pH calculator | Raise alkalinity or calcium |
| Too high (scaling) | Lower pH toward 7.2 to 7.4 | Lower alkalinity with the alkalinity calculator |
If your salt cell or heater keeps scaling, aim for an LSI slightly below zero at your warmest water temperature.
Frequently asked questions
What is a good LSI for a pool?
Between −0.3 and +0.3, with 0.00 ideal. Some pool pros aim for 0 to +0.3 in plaster pools to protect the surface.
What is the difference between LSI and CSI?
Both measure the same thing. The calcium saturation index (CSI) used in some Taylor guides adds corrections for ionic strength. For typical home pools the two give similar answers.
Why does the LSI change with temperature?
Calcium carbonate is less soluble in warm water, so heating makes the water more likely to scale. Cooling does the opposite.
Do I need to measure TDS?
It has a small effect. If you haven’t tested it, use about 1,000 ppm for a chlorine pool and 3,500 to 4,000 for a salt pool, since the salt itself counts as dissolved solids.
Why subtract CYA from alkalinity?
Test kits count cyanurate as alkalinity, but it doesn’t help form calcium carbonate. Leaving it in makes the LSI look higher than it really is.
My LSI is balanced but I still see scale. Why?
Scale often forms where water is hotter than the pool, like inside a heater or salt cell. Check the LSI at the heater’s outlet temperature, not just the pool’s.
Sources
- Understanding LSI: The Langelier Saturation Index. Orenda Technologies.
- Langelier Saturation Index (LSI). Mountain Empire Community College, water technology course notes.
- The solubilities of calcite, aragonite and vaterite in CO₂–H₂O solutions between 0 and 90°C (carbonate equilibrium constants). Plummer, L. N. and Busenberg, E., Geochimica et Cosmochimica Acta 46 (1982).
Reviewed November 7, 2026. We check the formulas against the sources above and re-run the worked examples whenever we update this page. Found something wrong? Tell us.