Langelier Saturation Index (LSI) Calculator

Q: How does temperature affect LSI?

Higher temperature increases LSI (makes water more scale-forming) because calcium carbonate becomes less soluble as water warms. The same drinking water that is balanced (LSI ≈ 0) in a 10 °C distribution main can become scale-forming inside a 60 °C residential water heater. This is why hot-water plumbing in hard-water areas scales much faster than cold-water plumbing.

WaterOperatorPracticeTest.com Updated May 21, 2026 Free >Free · Built by a licensed operator.middot; Built by a licensed operator

Free Langelier Saturation Index (LSI) calculator. The LSI — sometimes called the calcium-carbonate saturation index or water-aggressiveness index — tells you whether your water is undersaturated and corrosive, in equilibrium, or supersaturated and scale-forming with respect to calcium carbonate. Enter five basic water-quality values from your lab report — pH, temperature, total dissolved solids, calcium hardness, and total alkalinity — and you'll see the saturation pH (pH_s), the LSI value, and a plain-English interpretation instantly.

Pipe-interior comparison showing an uncoated corroded pipe versus a phosphate-protected pipe — the practical consequence of negative versus positive LSI

Inputs

pH units

Temperature

Total Dissolved Solids (TDS)

mg/L

Calcium hardness (as CaCO₃)

mg/L

Total alkalinity (as CaCO₃)

mg/L

Result

Show formula with your numbers

How LSI is calculated

LSI = pH − pH_s where pH_s = (9.3 + A + B) − (C + D)

The four sub-factors are:
A = (log₁₀[TDS] − 1) / 10 · B = −13.12 × log₁₀(T + 273) + 34.55 · C = log₁₀[Ca²⁺ as CaCO₃] − 0.4 · D = log₁₀[Alkalinity as CaCO₃]

Temperature in factor B is in degrees Celsius. The other inputs are in mg/L. pH_s is the saturation pH — the pH at which water would be exactly in equilibrium with solid calcium carbonate at your specific TDS, temperature, calcium, and alkalinity.

Interpreting your result

LSI < 0 — Water is undersaturated. It will dissolve calcium-carbonate scale and tends to be corrosive to metal pipes and fixtures.
LSI ≈ 0 (±0.3) — Water is in equilibrium with CaCO₃. Neither corrosive nor scale-forming.
LSI > 0 — Water is supersaturated. It will deposit calcium-carbonate scale on pipes, water heaters, and fixtures.

In practice, distribution systems aim for a slightly positive LSI (+0.1 to +0.5) to maintain a thin protective scale that isolates pipe metal from the water without causing excessive deposits.

When LSI isn't enough

LSI is a screening index — it tells you the tendency but not the rate. For more detailed analysis, operators also use the Ryznar Stability Index (RSI), the Aggressive Index (AI), and the Calcium Carbonate Precipitation Potential (CCPP). At very high TDS (>~10,000 mg/L) the simple LSI assumptions break down and a more rigorous calculation like the Stiff & Davis Stability Index is appropriate.

LSI vs. other water-stability indices

LSI is the simplest of several indices used to evaluate the calcium-carbonate stability of drinking water. Each has a slightly different scale and intended use.

Index	Formula	Interpretation	Best for
Langelier (LSI)	pH − pH_s	Negative = corrosive; 0 = balanced; positive = scale-forming	Drinking-water screening, certification exams
Ryznar (RSI)	2·pH_s − pH	<6 scale; 6–7 balanced; >7 corrosive	Cooling-water systems
Aggressive (AI)	pH + log(Ca·Alk)	<10 aggressive; 10–12 moderate; >12 non-aggressive	Asbestos-cement pipe evaluation
Stiff & Davis (S&DSI)	Like LSI, but ionic-strength corrected	Same scale as LSI	Seawater / brackish water (high TDS)
CCPP	mg/L CaCO₃ that would precipitate	Negative = will dissolve; positive = will deposit	Quantitative scale-rate prediction

Common operator mistakes with LSI

Mixing up calcium hardness with total hardness. LSI uses calcium hardness specifically, not total hardness. If your lab reports only total hardness, ask for a calcium hardness titration.
Using alkalinity as mg/L HCO₃⁻ instead of CaCO₃. All inputs to LSI use mg/L as CaCO₃. To convert HCO₃⁻ to CaCO₃, multiply by 0.82.
Forgetting to convert °F to °C. The B factor uses absolute temperature in Kelvin (T_°C + 273.15). The calculator's unit toggle does this for you.
Applying LSI to high-TDS water. LSI's TDS factor (A) is a rough approximation that fails above about 10,000 mg/L. Use the Stiff & Davis index for seawater, brackish water, or concentrated brines.
Reading LSI as a rate. LSI is a thermodynamic tendency, not a rate. Two waters with the same LSI can scale at very different speeds depending on kinetics.
Ignoring temperature stratification. LSI calculated at distribution temperature is balanced; the same water inside a 60 °C water heater is supersaturated. Always note which temperature you used.

When to use this calculator

Use it for distribution-system corrosion-control evaluation, for Lead and Copper Rule (LCR) decisions about pH and alkalinity adjustment, for studying the Level 3 and Level 4 operator certification exams (LSI questions are common at advanced levels), and as a quick check when blending two source waters to predict the stability of the mixed product. For pool and spa water, LSI is the industry-standard stability index — though the target range is wider (±0.5) than drinking water.

Frequently asked questions

What is a good LSI value for drinking water?

Most utilities target a slightly positive LSI between +0.1 and +0.5 in distribution. That maintains a thin protective calcium-carbonate scale on pipe interiors without depositing enough to clog mains or water heaters. An LSI below −0.3 is considered corrosive enough to trigger active corrosion control under the Lead and Copper Rule; an LSI above +0.5 typically causes excessive scaling and customer complaints.

Is a positive LSI bad?

Not by itself. A slightly positive LSI is generally desirable in distribution because it maintains a protective scale layer that isolates pipe metal from the water. The problem is excess: LSI above about +1.0 can deposit scale fast enough to clog small-diameter pipes, foul heat-transfer surfaces in water heaters, and produce visible white deposits on fixtures.

What is the difference between LSI and Ryznar Stability Index (RSI)?

Both estimate calcium-carbonate scale or corrosion tendency, but they're scaled differently. LSI = pH − pH_s and is symmetric around zero. RSI = 2·pH_s − pH and uses an absolute scale: under 6 indicates scale-forming, 6–7 balanced, over 7 corrosive. RSI is sometimes preferred for cooling water; LSI is the drinking-water and operator-exam standard.

How does temperature affect LSI?

Higher temperature increases LSI (more scale-forming) because calcium carbonate becomes less soluble as water warms. The same water that is balanced (LSI ≈ 0) in a 10 °C distribution main can become scale-forming inside a 60 °C residential water heater. This is why hot-water plumbing in hard-water areas scales much faster than cold-water plumbing.

Can LSI predict actual scaling rates?

Only directionally. LSI tells you the tendency — undersaturated, balanced, or supersaturated — but not the rate. Two waters with the same LSI can scale at very different rates depending on TDS, ionic composition, and the presence of natural scale inhibitors. For quantitative scaling predictions, use the Calcium Carbonate Precipitation Potential (CCPP) or run bench-scale recirculating tests.

What does LSI mean for the Lead and Copper Rule?

Under the LCR and its 2021 revisions (LCRR), systems exceeding the lead action level must implement corrosion control. LSI is one of several screening tools utilities use to evaluate whether pH or alkalinity adjustment is the right approach. A strongly negative LSI alone doesn't violate the rule, but it's a strong signal that lead and copper leaching from premise plumbing is likely.

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Disclaimer: This calculator is provided as a free study aid. Always verify treatment-plant decisions against a current bench-scale analysis and your state primacy agency's guidance under the EPA Lead and Copper Rule.