A new method for the direct least cost solution of pipe sizes of a branched pipe network of known geometry consisting of pumping station and network of pipes for known water consumptions incorporating various cost functions is being developed. The method developed gives optimum pumping head and total head loss in the network system and its optimum distribution among the various pipes of the network in seeking an overall least cost solution. The variation of the total cost with the total head loss in the system is studied. The method outlined herein is simple and can be efficiently used for the least cost design of branched water main systems only, with the help of a desk calculator. A systematic procedure of calculation has been described to illustrate the applicability of the technique using two sample problems.