A dishwasher appliance includes a sump that defines a transition fill level where the cross sectional area of the sump increases. A pressure sensor is operably coupled to the sump for monitoring sump pressure and wash fluid level. A controller opens a water supply valve to provide a flow of water into the sump and monitors the sump pressure using the pressure sensor. The controller determines when the water has reached the transition fill level corresponding to a known transition fill volume. The controller determines a fill flow rate based on the amount of time needed to reach the transition fill volume and uses the fill flow rate to determine a remaining fill time needed to reach a target volume.