Design procedure for an axi-symmetric magnetic field system is described. Coils having equal number of ampere-turns and identical radii are considered and their axial field is fitted to the desired field to obtain optimum positions and ampere-turns in the coils. The best fit parameters are obtained by means of chi-2-minimisation which is achieved using a simple algorithm involving method of steepest descent. The procedure is illustrated by means of the design for magnetic mirror field.