An analytical method for calculating an effective non-Darcy flow coefficientfor a heterogeneous formation is presented. The method presented here can beused to calculate an effective non-Darcy flow coefficient for heterogeneousgridblocks in reservoir simulators. Based on this method, it is shown that thenon-Darcy flow coefficient of a heterogeneous formation is larger than thenon-Darcy flow coefficient of an equivalent homogenous formation. Non-Darcyflow coefficients calculated from gas well data show that non-Darcy flowcoefficients obtained from well tests are significantly larger than thosepredicted from experimental correlations. Permeability heterogeneity is a verylikely reason for the differences in non-Darcy flow coefficients often seenbetween laboratory and field data.