Helix slow-wave structures have been proposed as the applicators for industrial microwave heating. The microwave characteristics of these applicators are calculated using an equivalent circuit model. Design formulae, based on the equivalent circuit model, are derived when the material to be heated is travelling along the helix axis. The formulae are simple and easy to use, and provide insight into the interactions between the helix dimensions and its microwave/heating characteristics. The design formulae can be used to determine the most suitable helix dimensions for heating a particular dielectric material. A design example is discussed for heating wool fiber at 2.45 GHz.