One of the most versatile types of heat exchanger used on process plants is the plate heat exchanger (PHE). It has principle advantages over other heat exchangers in that plates can be added and/or removed easily in order to change the area available for heat transfer and therefore its overall performance. The reason for changing the amount of heat transfer area may be down to a new duty requirement or non-replacement of damaged plates after maintenance. The following paper shows how the operation of a two stream PHE can be approximated after the plate rearrangement has been made, using the existing PHE performance data. This calculation is, therefore, one purely of rating with the assumptions that the overall mass flow rates do not change with the new configuration and that truly counter-current flow is achieved.