Large water distribution networks are divided into several small sub-networks called water supply zones. Each sub-network contains a central input point and a distribution system. There is nominal connection between the zones for inter-zonal transfer of water under varying demand patterns. Splitting of an area into zones is not only easy to design, it is also economical. Using geometric programming the present investigation determines the optimal zone size of circular and rectangular geometry. It is hoped that the present work will provide a guideline for deciding the zone size of an arbitrarily shaped area.