An approximate method to calculate the parallel neutron leakage in fast reactor slab lattices is described. It is derived from the integral transport equation and assumes isotropic scattering. By using an expansion in terms of oscillating functions, rather than the usual power series expansion in the buckling, it is proven that the method is also valid for voided cells. Results for a two-region cell are presented; they confirm that the widely used Benoist equation is valid for cases when sodium is present. However, for voided or nearly voided cells, the Benoist equation fails, whereas the new method is valid for any cell composition. The same method is applied to find the effective diffusion coefficient for a low-density channel. In the limit of zero buckling, the method reduces to well-known results available in literature by Rowlands. However, the buckling correction, obtained by a consistent expansion of the integral transport equation, is different from similar corrections in the literature.