LSI and RSI Calculator for Water Treatment Professionals
Our LSI and RSI Calculator provides a fast and reliable way to assess the scaling and corrosion potential of water in cooling towers, boilers, and other industrial systems. By entering a few standard field measurements, you can quickly determine whether the water is likely to form scale, promote corrosion, or remain within a stable operating range.
This tool is designed to support water treatment professionals by offering a structured, consistent way to interpret water chemistry—whether you’re assessing routine samples, reviewing laboratory results, or troubleshooting onsite variations. The calculator helps reduce guesswork, validate system trends, and complement your broader understanding of system design, metallurgy, and inhibitor performance.
LSI (Langelier Saturation Index) and RSI (Ryznar Stability Index) remain two of the most widely used indicators in the water treatment industry. When used together, they provide a clearer perspective on how the water is behaving relative to calcium carbonate saturation and long-term corrosive tendencies. This page includes guidance to help you interpret index values effectively, understand the factors that influence them, and apply the results in a practical operational context.
If your primary goal is simply to run calculations, you can skip straight to the tool below.
How to Use This Calculator & Required Inputs
This calculator uses standard field and laboratory measurements to determine the scaling or corrosive tendencies of system water. Before entering your results, please review the guidance below to ensure each input is entered correctly and in the correct units.
To use the calculator effectively, start by gathering the required water chemistry measurements and entering them into the fields provided:
- Prepare your measured pH, calcium hardness, alkalinity, temperature, and either TDS or conductivity.
- Enter each value carefully in the correct units, noting that TDS can be estimated from conductivity if needed.
- Click Calculate LSI & RSI to generate your results.
- Review the output values and compare them with the interpretation tables below for guidance.
- Use the indices as part of a broader assessment when evaluating system performance or selecting appropriate treatment actions.
Important considerations: LSI and RSI should be viewed as indicators only, as real-world system behaviour also depends on metallurgy, flow conditions, and historical data. Always ensure values are entered in the correct units—particularly calcium hardness expressed as CaCO₃. And avoid sampling from stagnant sections of a system, which can produce misleading results.
Tandex LSI & RSI Calculator
Fields marked with * are required to calculate LSI and RSI.
Tip: If TDS is left blank, it will be estimated from conductivity using TDS ≈ 0.64 × µS/cm.
Disclaimer: LSI/RSI indicate relative scale/corrosion tendency only and must be interpreted alongside metallurgy, system design, and operating history.
Understanding LSI and RSI
The Langelier Saturation Index (LSI) and Ryznar Stability Index (RSI) are two widely used indicators that help determine whether water is likely to form scale, remain balanced, or cause corrosion in industrial systems. Both indices evaluate how close the water is to the point where calcium carbonate becomes stable, taking into account pH, alkalinity, calcium hardness, temperature, and dissolved solids.
LSI compares the actual pH of the water to its theoretical saturation pH (pHs) to estimate how likely calcium carbonate is to precipitate or dissolve within the system. Positive values indicate a tendency toward scale formation, while negative values suggest conditions that may promote corrosion. These tendencies help assess whether the water is chemically balanced or requires adjustment.
RSI uses the same input values but expresses the outcome as a stability rating that reflects the water’s long-term behaviour. Lower RSI values indicate greater potential for scale formation, while higher values indicate increasingly corrosive conditions. This rating provides a practical measure of overall system stability.
Together, these indices provide a practical and widely recognised method for assessing water stability across cooling towers, boilers, and closed-loop systems.
LSI and RSI give a good indication of whether water is leaning toward scale formation, sitting close to equilibrium, or becoming more corrosive. These values are helpful starting points, but they don’t tell the whole story. Real-world conditions—like how hot a system runs, how fast the water is moving, the type of metals in contact with the water, system cleanliness, and how well the treatment program is performing—can all influence how water behaves compared with its calculated index values.
A single LSI or RSI result can point you in the right direction, but trends over time are far more important. If results consistently shift toward scaling or corrosive conditions, that’s usually a sign that something has changed or deserves a closer look. Things like fluctuations in makeup water quality, biofilm activity, seasonal temperature changes, or operational adjustments can all impact the balance of the water.
It’s also worth remembering that LSI and RSI are indicators—not predictions. They won’t account for scale or corrosion that’s already present, how different treatment chemicals might interact, or what’s happening in areas of low flow or stagnation. Sampling errors, incorrect units, or temperature drift before measurement can also influence the numbers.
Think of LSI and RSI as tools that help build the bigger picture. They’re most useful when you’re reviewing system behaviour, checking the performance of a treatment program, or keeping an eye on longer-term trends. When results fall outside normal ranges—or when what you see onsite doesn’t match what the numbers suggest—it may be worth following up with visual inspections, corrosion monitoring, microbiological testing, or a review of the treatment approach.
If a deeper review of your results is needed, or if you would like assistance interpreting trends within your system, our specialists are ready to advise and support your decision-making process.
If a deeper review of your results is needed, or if you would like assistance interpreting trends within your system, our specialists are ready to advise and support your decision-making process.
