Ryznar Stability Index Calculator

Cooling towers, boilers and distribution pipes all care whether water lays down scale or eats metal. Enter five routine lab values and get the Ryznar Stability Index plus a plain-English read on scaling versus corrosion tendency.

TDS (mg/L)

Water temperature

Temperature unit

Calcium hardness (mg/L as CaCO3)

Alkalinity (mg/L as CaCO3)

Ryznar Stability Index

Item	Value

How the calculation works

Formulas
pHs = (9.3 + A + B) − (C + D) (Langelier saturation pH)
A = (log₁₀ TDS − 1) / 10 · TDS in mg/L
B = −13.12 × log₁₀(T + 273) + 34.55 · T in °C
C = log₁₀(calcium hardness as CaCO₃) − 0.4
D = log₁₀(alkalinity as CaCO₃)
RSI = 2 × pHs − pH

RSI value	Tendency
< 6.0	Scaling tendency — lower values, stronger scaling
6.0 – 7.0	Near stable
> 7.0	Corrosive tendency

Notes & limitations

The Ryznar index is empirical (Ryznar, 1944), built on the Langelier saturation pH. It predicts tendency, not rate or quantity — confirm with coupons or field observations before acting.

RSI below 6 means scale is likely; the further below 6, the stronger the tendency. It does not tell you how much scale will form — pair it with the CCPP calculator for quantity.
RSI above 7 means the water is undersaturated and will not lay down a protective CaCO3 film; consider pH/alkalinity adjustment or a corrosion inhibitor under your corrosion control plan.
Inputs must be in the stated units: calcium and alkalinity as CaCO3, TDS as mg/L. Results are sensitive to pH — use a calibrated meter and a fresh sample.
For public water systems, corrosion control decisions must follow state primacy agency requirements (for example, optimal corrosion control treatment under the Lead and Copper Rule).

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