Abstract The dimension of a well test indicates how conductance (the product of hydraulic conductivity and flow area) changes with distance from a pumping well. Dimensional information is very important because it is one of the few direct hydraulic measures of rock mass heterogeneity and connectivity. Although a relationship between flow geometry and well test dimension has long been hypothesized, little work has been done to evaluate this relationship, much less use it in practical fracture-flow problems. This paper provides examples of the variability of flow dimension in well test data sets from Swedish and Japanese experimental sites. The paper presents an approach which directly determines the dimension of a simulated fracture network, as observed from any point in the network. The approach uses a graph search algorithm to identify the connected portion of the network and calculate the distance-conductance relationships. These relationships translate directly into flow dimension.