Gravity or pumping water transmission lines are planned for a predetermined time horizon generally called design period. For a static population the lines can be designed, either for a design period equal to the life of the pipes sharing the maximum cost of the system or for the perpetual existence of the supply system. For a growing population or water demand, it is always economical to design the mains in staging periods and then strengthen the system after the end of every staging period. In the absence of a rational criterion, the design period of water supply mains is generally based on the designer's intuition, disregarding the useful life of the component sharing maximum cost, pattern of population growth or increase in water demand and discount rate. Provided herein are numerical criteria for the design period of water transmission mains.