Granular filters are used for protection of structures against scour and erosion. For a proper functioning it is necessary that the interfaces between the filter structure, the subsoil and the water flowing above the filter structure are stable. Stability means that there is no transport of subsoil material through the filter to the water above the filter, and there is no filter material removed by the currents above the filter. In principal, two types of granular filters can be distinguished, based on the two criteria enabling erosion: (1) base material can pass the pores in the filter material, and (2) hydraulic load is larger than threshold value: - geometrically sand-tight filters: no transport of base material is possible due to pores in the filter too small to allow base material to pass, -hydrodynamically sand tight filters: the hydraulic load at the interface is less than the threshold value of the base material. In the past various design methods have been published, amongst others by Wörman (1989) for a stable riprap protection at bridge piers without filters, and Bakker et al (1994) with respect to more general riprap protections without filters. All formulas are based on a limited number of tests. Recently, a desk study has been carried out focusing on two particular aspects: 1. interface stability as function of the thickness of the filter layer consisting of standard armour stone gradings, and 2. interface stability of gravel mixtures with a wide gradation. Based on a theoretical approach the study resulted in a new design formula for geometrically-open but hydrodynamically sand tight granular filter structures under currents. The new formula relates the required filter layer thickness to a characteristic diameter of the filter material taking into account the influence of the grading of filter and base material, the influence of turbulence and the damping of the hydraulic load in the filter. Laboratory experiments were carried out to validate the new formula. The paper presents the new design formula including the derivation. Furthermore, experimental set-up, test program and measurements of the laboratory experiments are presented.