Abstract The size of a irrigation unit command area (UCA) has significant influence on distribution of water among farms and the efficiency of irrigation within farms. A model incorporating the concepts of equity, efficiency and productivity is developed for designing optimal UCA size and is validated with data from Bhakra Canal System in India. The design of watercourses by adopting the suggested procedure would lead to improvement in three‐criterion parameters (equity, efficiency and productivity). This establishes the need for and scope of modifying the existing designs.