The permeability of a layer of closely placed concrete blocks on a sublayer of gravel or sand without a geotextile has been investigated. The results indicate that the permeability of the blocks can be described as a combination of the permeability of the joints and the permeability of the the gravel just below the blocks. The permeability of the joints can be described with the well known formulas for the pressure drop due to a sudden contraction and for the influence of wall friction. The influence of the gravel is described by formulas in groundwater flow computations. In this paper it is shown that a combination of these formulas leads to a permeability formula that agrees well with the results of permeability tests.