Within the context of multi-coil inductive power transfer (IPT) systems, the presence of mutual induction between same side coils has been a considerable challenge since it directly impacts system stability and performance. In this paper, a mathematical model is presented to accurately estimate the location of a null point in mutual inductance between two planar spiral circular coils. The impact of coil design parameters such as vertical separation, fill factor and outer radius are investigated for impact on null point location. This model is then verified against finite element analysis simulation results. The identified location of the null point is within a maximum of 0.39% error. The findings of this work are directly applicable to the design of multi-coil inductive power transfer systems where decoupling between same-side coils is advantageous to achieving high efficiency and power capability.