Sensitivity is the most important parameter of the Transponder coils from the performance point of view. Due to the open magnetic circuit constructions analytic formulas are not available and not applicable to calculate the sensitivity. Analogically to the self-inductance effective permeability a similar effective permeability can be introduced for the mutual inductance, as well. The presented sensitivity calculation method is based on the mutual effective permeability. By the presented method, the sensitivity of inductors with arbitrary shape and permeability materials can be calculated with good accuracy.