Langelier Saturation Index (LSI) Calculator
Use this tool to estimate whether water is likely to be scale-forming, balanced, or corrosive based on common field measurements.
Formula used: LSI = pH − pHs, where pHs = (9.3 + A + B) − (C + D)
What the Langelier Index tells you
The Langelier Saturation Index (LSI) is a practical water chemistry metric used in pools, cooling towers, municipal systems, and industrial loops. It estimates calcium carbonate stability in water. In plain terms, it helps answer: Will this water deposit scale, stay balanced, or corrode surfaces?
LSI does not directly measure corrosion rate or scale thickness. Instead, it predicts chemical tendency based on pH, temperature, dissolved solids, calcium hardness, and alkalinity. It is best used as a decision aid alongside regular testing, material compatibility checks, and operating history.
How this calculator works
Inputs required
- pH: your measured water pH.
- Temperature (°C): water temperature during sampling.
- TDS (mg/L): total dissolved solids concentration.
- Calcium Hardness (as CaCO₃): calcium hardness converted to CaCO₃ units.
- Total Alkalinity (as CaCO₃): alkalinity in CaCO₃ units.
Core equations
The calculator uses a common field equation set:
- A = (log10(TDS) − 1) / 10
- B = −13.12 × log10(Temperature in Kelvin) + 34.55
- C = log10(Calcium Hardness as CaCO₃) − 0.4
- D = log10(Total Alkalinity as CaCO₃)
- pHs = (9.3 + A + B) − (C + D)
- LSI = pH − pHs
How to interpret your LSI value
| LSI Range | General Interpretation | Typical Action |
|---|---|---|
| > +0.5 | Scale-forming tendency | Reduce scaling risk by adjusting pH/alkalinity/calcium as needed. |
| 0.0 to +0.5 | Slight scale tendency / near balanced | Monitor and fine-tune; often acceptable in many systems. |
| -0.5 to 0.0 | Slightly corrosive tendency | Watch metal surfaces and review treatment targets. |
| < -0.5 | Corrosive tendency | Investigate promptly; rebalance chemistry and inspect equipment. |
Practical guidance for better results
1) Use consistent units
Calcium hardness and alkalinity must be entered as mg/L as CaCO₃. If your test kit reports different units, convert before calculation.
2) Sample correctly
Measure pH and temperature immediately at the sample point. Delays, aeration, and warming/cooling can shift pH and distort your index value.
3) Track trends, not single points
One reading is a snapshot. A weekly trend line gives stronger operational insight, especially in systems with fluctuating makeup water or variable loads.
4) Pair LSI with inspection data
Always combine calculated risk with field evidence: heat exchanger approach temperatures, filter pressure changes, visible deposits, coupon data, and corrosion monitoring.
Common adjustment strategies
- If LSI is too high (scaling): lower pH gradually, review calcium concentration, and avoid overcorrecting alkalinity.
- If LSI is too low (corrosive): increase alkalinity or pH carefully and confirm corrosion inhibitor program performance.
- If near zero: continue routine monitoring and keep chemistry within your site’s target control band.
Limitations you should know
LSI is a useful screening tool, but it does not capture all chemistry behavior. High chlorides, sulfates, disinfectants, flow regime, metallurgy, and biofouling can influence outcomes independently. In critical systems, use LSI as one component in a broader water treatment program.
Quick FAQ
Is LSI only for pools?
No. It is widely used in potable water, industrial cooling, boilers (with caution), and process water management.
What is an ideal target?
Many operators aim close to 0, often within a narrow operating band (for example, around -0.2 to +0.2), but the best target depends on your equipment, materials, and regulatory requirements.
Does a positive LSI always mean severe scale?
Not always. It indicates tendency, not guaranteed deposition rate. Residence time, heat transfer surfaces, velocity, and inhibitors all matter.