Cold-water supply system and air-conditioning air distribution system are very important in providing services in residences, commercial buildings and industrial plants. Yet, in practical scenes, it is quite often seen that these systems fail to perform as well as expected. The main problems are the improper design of the water piping and air-conditioning ducting systems concerned. The objective of the project is to develop a computer software as a powerful tool for mechanical designers to determine the sizes of cold-water piping and air-conditioning ducting systems efficiently. Various considerations have been made, such as fluid flow rate, mean velocity, length, fittings, fixtures, standard sizes recommended, head loss, head gain and the economic factors. For cold-water piping system, the Estimating demand method for Various Pipes Water Supply Design has been chosen and for air-conditioning ducting system, the Expedited Equal Friction Method. The software developed is reliable, user-friendly with error-checking messages and provided with help hotkeys.