ESDU FMI1 provides guidance on simplified methods of calculating pressure losses in internal flow systems. It assists in the use of a series of Data Items in Volumes 1-4 of the Fluid Mechanics@ Internal Flow@ in which detailed numerical data are given. It also provides a brief summary of the complete contents of the Internal Flow Sub-series. The Item covers a wide range of practical internal flow problems@ but it is restricted to the steady@ single-phase flow of Newtonian fluids.