Total and/or hot water consumption has been measured in a number of apartment buildings of different sizes in Belgium. Each building was monitored during a minimum period of 1 month with a measurement interval of 1 second. Peak flow rates were deduced from these data for each building. The measured peak flow rates were compared to the peak flows as calculated in accordance with existing guidelines in order to determine the relevance of these guidelines for Belgium. Furthermore, the impact of measurement interval on peak flow rate was examined by averaging the data over longer intervals. Although more measurements will be needed to obtain statistically relevant results, existing guidelines seem to overestimate the peak flow rate by a factor of at least 2, and up to 3. Averaging of measuring data over intervals up to 5 seconds had no significant influence on the measured peak flow rate. However, it should be noted that measuring over an interval of 60 seconds would underestimate the peak flow rate significantly. A new approach using semi-logarithmic graphs is suggested to illustrate the correlation between maximum flow rate data and measuring intervals.