Langelier Saturation Index (LSI)

Also known as LSI · calcium carbonate saturation

\mathrm{LSI} = \mathrm{pH} - \mathrm{pH_s}

Worked example: pH 7.80 against pHs 7.31 → LSI = +0.49 (scaling). Press Try an example to run it live, then adjust anything.

Enter your known values, leave one input blank, and solves for the missing one. Tap a variable’s symbol to see what it means, with a typical value.

Where Langelier Saturation Index (LSI) fits

Missing an input?

pHpH from Hydrogen Ion Concentration · pH and pOH Relation
pHsSaturation pH (pHs) for Langelier's Index · Saturation pH (pHs), Rigorous: from the Carbonate Equilibria

In this toolkit

Scaling and corrosion indices6 formulas

Langelier Saturation Index from a Full Water Analysis: The index in one step from a water analysis, with the rigorous saturation pH and the alkalinity corrected for pH.Cooling Tower LSI at Cycles of Concentration: The same index at cooling-tower conditions: the makeup cycled up, at the hottest surface.Ryznar Stability Index (RSI): Both use measured pH and saturation pH.Predicted Cooling Tower LSI and Maximum Cycles, from the Makeup Analysis: Both use langelier Saturation Index.Puckorius (Practical) Scaling Index: Both use saturation pH.Saturation pH (pHs) for Langelier's Index: Both use saturation pH.

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Langelier Saturation Index (LSI) explained

Wilfred Langelier published his index in 1936 to answer one question every plant operator still asks: will this water lay down calcium carbonate, or will it eat my pipe? The index is nothing more than the distance between the pH you measure and the pH at which the water would be exactly saturated with CaCO₃. A positive LSI means the water is supersaturated and will deposit scale; a negative LSI means it is hungry and will dissolve any protective carbonate film it finds, exposing bare metal. Zero is theoretical equilibrium. A cooling tower running pH 7.8 against a saturation pH of 7.31 sits at LSI = 7.8 − 7.31 = +0.49, comfortably scaling and in need of acid or a scale inhibitor.

The trap is treating LSI as a rate. It is a direction only: a thermodynamic yes/no, not a mils-per-year number. Two waters can both read +0.5 and behave completely differently because one carries 400 mg/L of calcium and the other 40. That is why field practice pairs LSI with the Ryznar index and, in cooling systems, with a calcium-times-alkalinity limit. Also remember that LSI is a bulk-water number: heat exchanger skin temperatures run 10–20 °C above the bulk, and a water that is −0.2 in the basin can be +0.6 on the tube wall.

Langelier Saturation Index (LSI) formula

\mathrm{LSI} = \mathrm{pH} - \mathrm{pH_s}

Where

$\mathrm{LSI}$ = Langelier Saturation Index
$\mathrm{pH}$ = Measured pH
$\mathrm{pH_s}$ = Saturation pH

Where this comes from

Langelier, W. F. (1936). The analytical control of anti-corrosion water treatment. Journal AWWA, 28(10). Read it
Standard Methods for the Examination of Water and Wastewater (APHA, AWWA, WEF), Section 2330: Calcium Carbonate Saturation.

In this toolkit

Scaling and corrosion indices

6 formulasLangelier, Ryznar, Puckorius, Larson–Skold and coupon corrosion rate — the indices that say whether water will scale or eat the pipe.

Revised 31 October 2026

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formula.expert. (2026, October 31). Langelier Saturation Index (LSI) Formula & Calculator. https://formula.expert/solvers/langelier-saturation-index

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