Experimental and Computational fluid dynamics (CFD) studies of the technique Of fluid dynamic gauging (FDG) in duct flows have been performed. Experiments were performed using a stainless steel gauging nozzle located on the centreline of a Perspex duct of square cross section. The test fluid was water, flowing through the duct at 0.0077-0.74 m/s (Re-duct 116-11 400). The success of the experiments was confirmed by the results of Tuladhar et al. [2003. Dynamic gauging in duct flows. Canadian journal of Chemical Engineering 81, 279-284]. For the first time, CFD has been applied to simulate FDG in an imposed flow for steady, incompressible, laminar flows. Experimental data and simulation results agreed to within 6%, supporting the validity of both the experiments and the assumptions underpinning the simulation. CFD simulations predicted the stresses beneath the lip of the nozzle and confirmed the practical working range of the gauge (0.1 < h/d(t), < 0.25). This is a major achievement, proving that CFD can be used to model this flow-FDG accurately, which is Valuable for future work in this area, namely fouling in food, crude oil and cross-flow membrane systems. (C) 2008 Elsevier Ltd. All rights reserved.