Langelier Saturation Index (LSI) Calculator

Introduction

For RO plant engineers who need a fast, auditable scaling-versus-corrosion read on a water analysis — feedwater, concentrate or permeate — before deciding on antiscalant dosing or pH correction, using feedwater TDS, temperature and the CaCO3-based hardness and alkalinity figures already on the lab sheet. The Langelier Saturation Index (LSI) and Ryznar Stability Index (RSI) are crucial metrics. This calculator determines the Langelier Saturation Index LSI = pH - pHs from TDS, temperature, calcium hardness and alkalinity, with the Ryznar index RSI = 2 pHs - pH from the same pHs.

How to Use This Calculator

1. Enter the measured pH of the water sample into the pH field.
2. Enter TDS in mg/L and the water temperature in °C.
3. Enter calcium hardness and total alkalinity, both in mg/L as CaCO3, exactly as reported by the lab — do not convert.
4. Read pHs, LSI and RSI from the results block; LSI = pH − pHs, so a positive LSI flags scale-forming water and a negative LSI flags corrosive water.
5. Check the A, B, C, D breakdown under the results to confirm which term is driving the index.

FAQ

Q: What LSI value means the water is scale-forming or corrosive?

A: LSI = pH − pHs, so a positive LSI means the water is scale-forming and a negative LSI means it is corrosive; a value near zero means the water sits close to calcium carbonate saturation.

Q: How is pHs calculated in the Langelier Saturation Index?

A: pHs = (9.3 + A + B) − (C + D), where A = (log10(TDS) − 1)/10, B = −13.12·log10(T + 273) + 34.55, C = log10(calcium hardness) − 0.4 and D = log10(alkalinity). The calculator shows each of these four terms alongside the result.

Q: How do I get the Ryznar Stability Index (RSI) from pHs?

A: RSI = 2·pHs − pH, using the same pHs computed from TDS, temperature, calcium hardness and alkalinity; this calculator outputs it automatically. As a guide, an RSI below about 6 is scale-forming and above about 7 is corrosive.

Q: What units do calcium hardness and total alkalinity go in for the LSI formula?

A: Both must be entered in mg/L as CaCO3; TDS goes in as mg/L and temperature in °C, because the formula applies log10 to the hardness and alkalinity and adds 273 to the temperature to work in kelvin.