In packet-switched data networks, overheads are incurred primarily due to two aspects: the protocol, i.e. headers. acknowledgements etc. and blank padding of the partially used packet. In the letter, the overhead factors due to these two sources are derived under certain assumptions. Simulations using exponentially distributed messages have been carried out, and results from these are also presented. It is shown that, under certain conditions, it is possible to have a packet size that minimises the total overheads.