The term lacus generally identified the public fountains in the main streets of ancient Roman towns, providing for the population daily water demand. The simplest lacus consisted of a stone basin and a spout stone, concealing one or two supply pipes. 35 street fountains of this type have been surveyed in Pompeii, to gather information on their supply and its variation in time. A new method was devised for calculating the discharge through the overflow channel of each lacus, and this value was taken as an estimate of the water supplied to each fountain. The overflow channel internal cross-section width was measured at four elevations, and the cross-section profile was reconstructed based on these data. Three water levels of 1 cm, half of the cross-section height and entire cross section height, were considered at each channel’s inlet, obtaining a corresponding channel discharge. The values obtained, ranging from 0.03 to 2.9 l/s, were checked against the trajectory of the fountain water jet, making sure that it remained within the basin length. For 28 fountains the average discharge was found to be 0.08 l/s when the water was at the lowest level, 0.43 l/s for the intermediate level and 1.18 l/s for a full inlet. The average time of residence of the water, in the lacus draw basin, was estimated between 11 min and 3 h. An estimate of the demand of all the town lacus was compared with the capacity of the aqueduct channel entering at Porta Vesuvio: the town lacus could have been supplied contemporaneously at the minimum and intermediate discharges.