Washing machine appliances and methods for operating washing machine appliances are provided. A method includes calculating a hot water fill time and a cold water fill time based on an assumed hot water temperature, an assumed cold water temperature, an assumed hot water flow rate, an assumed cold water flow rate, and a desired wash water temperature. The method further includes actuating a hot water valve to flow hot water for the hot water fill time, and actuating a cold water valve to flow cold water for the cold water fill time.