Langelier Saturation Index (LSI) Calculator
Use this tool to estimate whether water is likely to be scale-forming, balanced, or corrosive. Enter measured water values below.
Formula used: LSI = pH - pHs, where pHs = (9.3 + A + B) - (C + D).
What is an LSI calculator?
An LSI calculator estimates the Langelier Saturation Index, a water chemistry indicator used in pools, spas, cooling systems, municipal water treatment, and plumbing design. The index predicts how calcium carbonate behaves in water:
- Positive LSI: water tends to deposit scale.
- Near zero LSI: water is near equilibrium.
- Negative LSI: water tends to dissolve calcium carbonate and can become corrosive to surfaces and equipment.
Why LSI matters
If your water is too scale-forming, you may see rough plaster, cloudy water, heater inefficiency, and clogged lines. If it's too aggressive, it can etch plaster, damage metal components, and shorten equipment life. LSI gives you one quick number to evaluate whether your chemistry is trending in a safe direction.
How this lsi calculator works
This calculator uses the classic Langelier approach and computes a saturation pH (pHs) from five measured parameters:
- Measured pH
- Water temperature
- Total dissolved solids (TDS)
- Calcium hardness
- Total alkalinity
A = (log10(TDS) - 1) / 10
B = -13.12 × log10(°C + 273) + 34.55
C = log10(Calcium hardness) - 0.4
D = log10(Alkalinity)
pHs = (9.3 + A + B) - (C + D)
LSI = pH - pHs
General interpretation ranges
- LSI > +0.5: strong scaling tendency
- +0.2 to +0.5: mild scaling tendency
- -0.2 to +0.2: typically balanced
- -0.5 to -0.2: mildly corrosive tendency
- < -0.5: corrosive/aggressive tendency
How to use this tool correctly
1) Use fresh test data
Old water test readings can produce misleading results. Always retest before making dosing decisions.
2) Keep units consistent
Enter calcium hardness and alkalinity as mg/L as CaCO₃. Incorrect unit conversion is one of the most common causes of bad LSI results.
3) Treat LSI as a guide, not an absolute rule
LSI is powerful, but it does not capture every variable (such as specific metal content, sanitizer residuals, flow conditions, and specialty surfaces). Use it together with routine operational checks.
Practical adjustment strategy
When LSI is outside target range, operators usually adjust chemistry in this order:
- pH: fastest way to move LSI up or down.
- Alkalinity: supports pH stability and shifts saturation balance.
- Calcium hardness: important for plaster protection and scaling control.
- Temperature/TDS: less controllable in many real systems but still part of the balance.
Example scenario
Suppose a pool test shows pH 7.6, temperature 25°C, TDS 500 mg/L, calcium hardness 220 mg/L, and alkalinity 100 mg/L. The calculated LSI will usually land slightly negative to near balanced depending on exact conditions. That indicates minimal scaling risk and limited corrosive pressure, often acceptable for normal operation.
Frequently asked questions
Is a higher LSI always better?
No. High positive LSI can cause scale and cloudy water. Most operators target a narrow, near-zero range.
Can I use this for drinking water systems?
Yes, as a screening tool. For regulatory or engineering decisions, use certified lab results and site-specific standards.
Does sanitizer level affect LSI?
Not directly in the equation shown here, but sanitizer chemistry can influence pH trends and surface behavior over time.
Bottom line
A reliable lsi calculator helps you quickly evaluate water balance and prevent costly damage from scale or corrosion. Use the calculator above with accurate test values, keep your LSI near equilibrium, and review chemistry routinely for long-term system health.