Reed switch manufacturers specify activation coercivity (H). This is generally associated with solenoidal coils to calculate amp turns required to activate the reed switch. A permanent magnet is also used to generate the required field, but the distance between the permanent magnet and reed switch is usually a trial and error process starting with a reed switch and knowledge gained from experience with different magnets. This does not allow one to anticipate the factors involved in selecting the optimum dimensions for any particular combination of magnet and reed switch.

The following calculation is the only known closed form solution for the coercivity (H) normal to the magnet as shown in the figure. The magnetic field coercivity calculation applies in the absence of the reed switch. The presence of the reed switch modifies the field. The closure or activation of the reed switch further modifies the field which is the origin of the distance hystersis to the first order of magnitude. This calculation provides a first order approximation of distance in relationship to permanent magnet dimensions. Finite Element Magnetics are required for more accurate field calculations.