Up to now the prediction of separation efficiency for disk stack separators is largely based on the concept of equivalent clarification area, short sigma-theory, firstly published in 1949. An improved analytical model with a closed-form solution is presented in which the assumptions of uniform flow and unhindered solids transport are dropped. The contributions of distributor and solids holding space to the total separation efficiency are considered. The model is validated by comprehensive experiments with different geometry parameters and particle systems. The results are presented in a universal form for a combination of dimensionless numbers.