Diffusion and sorption in the rock matrix are important retardation mechanisms in radionuclide transport in fractured media. The surface area in contact with the flowing water so called flow-wetted surface controls the interaction with the rock matrix. The flow-wetted surface, FWS, may be determined from the frequency of open fractures intersecting a borehole. The choice of the packer distance used in these hydraulic measurements is however crucial. If the packer distance is large, several open fractures may be found in the same packer interval. Analytical solutions and numerical calculations are used in order to address the determination of the flow-wetted surface from borehole data. The study is focused in two important aspects of the fieldwork, namely the distance used between the packers and b) the number of measurements that are required in order to obtain a accurate determination of the flow-wetted surface. A large volume of hydraulic data in Sweden obtained by using quite small packer distance is revised. The most usual packer distance was 3 meters. However, for some boreholes or sections of them even smaller packer distances were used. From these data, the flow-wetted surface was determined at several sites in Sweden. In some sites, the used packer distance was too large that no accurate determination of the flow-wetted surface could be done.