Estimation of the permeability tensor is crucial to many natural and industrial porous media applications. Homogenization provides a rigorous approach to calculate effective parameters from pore-scale images of porous media by solving a boundary value problem on a representative unit cell. However, it requires the enforcement of global constraints that are hard to satisfy for arbitrarily complex geometries. We have developed a novel computational algorithm to rigorously calculate the permeability tensor. We show that the proposed algorithm has high accuracy.