AbstractThe evaluation of water loss level in water supply systems through performance indicators is a crucial issue, which has long been debated. The Infrastructure Leakage Index (ILI) is derived from the structural and operational characteristics of the network. Some parameters, such as the length of service connections, may be difficult to estimate; furthermore, the strong dependence of ILI on the average operating pressure (AOP) makes it sensitive to the way this parameter is defined, making it potentially unsuitable for comparing different systems. In this work, a methodology for the calculation of ILI in large water systems which may exhibit a non-homogeneous distribution of the parameters is proposed. Great relevance will be given to the assessment of the AOP, based on data available both by measurements and numerical modeling. A sensitivity analysis of ILI with respect to the parameters will be carried out as well.