The water flow rate of a pump can be calculated using other measurable inputs along with calibrated pump characteristic curves. Head-based and power-based virtual pump water flow meters apply the affinity laws to convert the calibrated pump head and power curves at the rated frequency to the curves at measured frequency and then calculate the water flow rate using measured head and power respectively. Due to the mismatched correlation between the motor speed and input frequency, these two flow meters cannot achieve acceptable accuracy. The purpose of this paper is to improve the accuracy of these two flow meters using the calibrated head and power curves at multiple frequencies rather than at the rated frequency only. First the mathematical models of flow measurements and uncertainty analysis are introduced, then two experiments are conducted, one for the virtual meter calibration and uncertainty analysis while another for the accuracy improvement validation. The results show that the uncertainty of water flow measurements decreases from 13 m(3)/h to 5 m(3)/h for the head-based flow meter and 17 m(3)/h to 6 m(3)/h for the power-based flow meter if the calibrated pump characteristic curves at multiple frequencies replace the curves at the rated frequency. (C) 2019 Elsevier B.V. All rights reserved.