Transcranial magnetic stimulation (TMS) allows us to excite or inhibit the function of the brain non-invasively. The distribution of the electric field induced in the brain, and the affected brain areas, strongly depend on the individual anatomical features of the whole head and the gyral and sulcal geometry. This paper proposes a computational method that is based on the finite-element and geometric multigrid methods for fast calculations of the induced electric field. Using the proposed method, it becomes possible to determine the electric field accurately in a matter of seconds, enabling individualized electric field simulations for human brain research.