A system for bandwidth estimation includes an interface and a processor. The interface communicates with a fat-tree (FT) network including multiple switches including (i) leaf switches belonging to a bottom level, (ii) spine switches belonging to a top level and (iii) intermediate switches belonging to one or more intermediate levels. Links connect between selected ones of the switches. The processor is to calculate, for a given level of the FT network that is divided into multiple groups of switches, oversubscription ratios for the respective groups, an oversubscription ratio of a group being indicative of a ratio between (i) a first available bandwidth on the links connecting the switches in the group to a lower level, and (ii) a second available bandwidth on the links connecting the switches in the group to a higher level, and to report a figure of merit of the FT network based on the oversubscription ratios.