Electrostatic probe measurements taken under continuum conditions in a flowing ionized gas can overestimate the charged‐particle density by several orders of magnitude if the convection of charged particles into the plasma sheath is not included in the analysis. A simple physical model which includes both diffusion and convection currents is described, together with an experiment in which charged‐particle densities inferred from spherical and disk probes were compared to microwave electron density measurements. The experiment was performed in a room‐temperature flowing afterglow plasma over a Reynolds number range 0.4–70 under thick‐plasma‐sheath conditions. The agreement between the microwave and the spherical probes' ion current measurements was within 30%, and the ion current‐voltage characteristic was described to better than 10% at both a high and low Reynolds numbers. The interpretation of electron current data is also discussed.