The paper deals with a procedure for the computation of the magnetostatic field in a vacuum. A very short summary of some usual methods for the field computation is given first, recalling their advantages and drawbacks, then a mixed analytical-numerical method is presented, which makes use of the analytical solution of Biot-Savart's integral for simple elementary space domains. The method permits to obtain an accurate solution for a wide set of field source geometries, even when the field is computed inside the sources. In the end, examples of application of the method are presented.