Lime-soda softening is one of the oldest and most important industrial water treatment processes. However, few engineers and chemists can accurately predict effluent quality that will be obtained by this type of softening. Predictions are poor even when computers are used to perform precise and involved chemical equilibrium calculations. This paper offers a different approach for improving water softening calculations based on the use of operational solubility product constants (OSPCs) as a way of bridging the gap between chemical theory and actual practice. To demonstrate the effectiveness of the OSPC approach, this paper describes how operational solubility product constants are determined and used with a new microcomputer codes, SOFTEN (1), and presents comparisons of treated water compositions predicted with OSPCs and compositions measured in laboratory and field studies.