A new method is proposed for calculating the inductivity of single-layer hollow coils which takes into consideration the spiral shape of the coil, the discontinuity and the effect of the leads. The equivalent dimensions are determined by using a one dimensional model with constant current density. A new inductance factor is determined in place of the Nagaoba factor. The values obtained by the new method show very good agreement with measurement results. The method is applicable to dimensioning inductor change systems in heating, air transformers and reactors.