In this paper we present an approach to calculate the time-varying equivalent inductance and resistance of helical flux compression generators accurately. This approach is based on the 2D filamentary method and uses the dynamic matrix concept. By using this method, it is possible to consider both the effects of the armature presence and frequency on the inductance and resistance of the HFCGs. The latter effect has not been discussed in previous open literature. The validation of the presented method has been verified by measurement results.