INTRODUCTION This Memorandum is concerned with the calculation of frictional pressure losses associated with the flow of fluids in straight pipes running full. Data are given for both laminar flow and turbulent flow for a range of values of surface roughness. Both circular and non-circular cross sections are considered. The information given covers a large proportion of practical cases@ but there are limitations to its validity and these are discussed in Section 7. Two methods of solution of pipe flow problems are outlined. Their distinguishing features are I formulae for calculating the friction factor (Section 3.2)@ II a friction factor - Reynolds number chart (Figure 1). Guidance is given on the most appropriate choice of method for various types of problem. In addition@ Appendix A introduces a computer program@ provided on disk in the Software Volume@ that calculates any of the following parameters@ assuming that the other three are known: (i) pipe cross-sectional dimension (or ratio of dimensions)@ (ii) pipe length@ L@ (iii) volume flow rate@ Q@ (iv) and pressure loss in duct@ Dp. The program evaluates the friction factor from the more-recently developed Chen equation (Reference 26).