ESDU 81039 provides basic data for use in the calculation of pressure losses for incompressible flow (generally liquid flow) in ducts containing orifice plates, perforated plates and thick orifice plates or in the determination of a suitable geometry for a pressure reducing orifice in a duct. Data are given for porosities in the range 0.001 to 1.00; although the data were derived basically for symmetrical arrangements of circular holes in circular plates they were found to apply reasonably well for square ducts, square holes and small departures from symmetry. General features of flow through orifices are discussed. A flowchart calculation procedure is included. Compressible flow is dealt with in ESDU 82009.