ESDU 81021 provides a means of estimating pressure losses caused by bluff obstructions in pipe and duct systems, such as a pipe crossing a duct, an instrumentation housing projecting into a pipe or equipment obstructing a mine gallery. The data cover bluff bodies of flat plate, wedge, rectangular block or circular-cylindrical sections in ducts or pipes of all cross-sections, for liquid flow and incompressible gas flow. An Appendix briefly discusses the derivation of, and causes of uncertainty in, the data.