An equation in integral form has been derived which in principle enables the calculation of the magnetic field associated with any simple magnetron whose geometry may be regarded as being two dimensional. Some approximations are necessary in the course of the analysis but these are not unduly restrictive. The equation is arrived at using conformal mapping techniques. It is used to find analytic expressions for the magnetic stream function and the magnetic field for the "standard" rectangular magnetron geometry. Theoretical results are given for a commercial planar magnetron system. These are then compared with experimental observations.