Grinding circuit optimisation is a complex problem requiring suitable decision aids. Both the need to reduce experimental costs and the improving reliability of mathematical modelling are leading to a growing acceptance of computer simulation methods by engineers in mill design and grinding practice. Using a minimum parameter grinding model for tumbling mills, the authors have developed the ＇＇reference state method＇＇ for the calculation of grinding systems. The method enables prediction of system behaviour when changes occur in the most important variables and parameters. A new ＇＇mill efficiency curve＇＇ analogous to the normalised grade-efficiency curve for describing classifier behaviour, is suggested. It describes a unique size-dependent mass flow through the mill to obtain a certain fineness condition. The curve is relatively invariant over a reasonable range of conditions. It is excellently suited for objective evaluation and simulation. The wide application range of the method is illustrated by typical examples, showing the advantages both of the method and of the corresponding complex software solution ＇＇CGC - Calculation of Grinding Circuits＇＇.