The applicability of the focused-beam reflectance measurement (FBRM) device for particle-size measurements depends on an appropriate transformation of the measured chord lengths to particle sizes. This problem has received considerable attention in the recent past. However, most of this effort has been directed toward either spherical particles or nonspherical particles of uniformly distributed orientations. In actual practice, the particle orientation must be biased by the flow of the fluid in which the particles are suspended. Such orientation bias becomes particularly serious for highly nonspherical particles such as those of needle shape that occur frequently in the pharmaceutical industry. This paper addresses the significant problem of transforming chord lengths of such particles with orientation bias to their sizes. Our approach is predicated on the solution of an inverse problem for extracting the orientation bias by measuring the chord-length distributions of particles whose size distributions are known. As the flow in most process equipment will be a function of position, the orientation bias will depend on the site of measurement. Thus, such a calibration approach will be specific to particle shape and measurement site because the flow conditions will vary with location. The proposed methodology is the first of its kind, which we expect to be a promising approach to fully exploit the simplicity of the FBRM device.