Granular filters are used for protection against scour and erosion of base material. For a proper functioning it is necessary that at the interfaces between the filter structure, the subsoil and the water flowing above the filter structure no material will be transported. Different types of granular filters can be distinguished, this paper focuses on stable geometrically open filter structures under current attack. Hoffmans (2012) developed a design formula for stable geometrically open filters. This paper presents the validation and an optimization of the design formula based on performed model tests. It is shown that the current design formula is too conservative. The proposed improvements allows for a wider range of applicability.