SMEGREG is a public body in charge of water saving in the Gironde département (French administrative unit) in southwest France. SMEGREG has devised its own information system to collate water balance component data from all utilities in the region. Analysis of those data highlighted numerous disparities in the methods used by different utilities to calculate annual volumes. At the request of SMEGREG, Irstea carried out a detailed study of the practices of five drinking water utilities. The process used to estimate volumes is referred to as "annualisation". The annualisation process can be broken down into three main steps: 1) Determining a representative date for Customer meter readings, 2) Adjusting volumes measured by main meters, 3) Adjusting metered consumption to fit a calendar year. There are several possible methods of carrying out each of these steps. In this research project, all methods were applied to each utility under study. Calculations show that annualisation methods have a strong impact on volume estimates. At the end of this study, SMEGREG made two strong recommendations to data providers: 1) Annualisation methods must be identified, and all calculations must be reproducible, 2) The same combination of methods should be used every year.