A new useful index for predicting the maximum safe operating cycles of concentration in ozonated cooling systems is reported. This index utilizes simple, easily obtainable makeup water quality parameters including conductivity, alkalinity, calcium and magnesium hardness, chloride, and sodium and is easily calculated. The index provides greatly improved correlation with real-world-derived scaling tendencies in ozonated cooling systems compared to conventional scaling indices such as the Langelier Saturation Index (LSI), the Ryznar Stability Index (RSI), and the Puckorius Stability Index (PSI). A simplified version of the index utilizing only total hardness also can be used to establish quickly maximum operating cycles of concentration in well characterized, but very rapidly changing water supplies.