Flowrate is one of the most important process quantities measured in the water industry. To accurately measure flowrate, the flowmeters have to be calibrated regularly. An efficient method presented herein is used to calibrate flowmeters quickly for ensuring an accurate measurement of flowrate in the water distribution system. It is based on the regularity in open-channel flows that the ratio of the mean to maximum velocity in a given cross section is constant. The maximum velocity can be easily determined on a vertical, which tends to remain invariant with time and flowrate, and can be converted to the mean velocity of the cross section by this constant ratio. The most commonly used method, the velocity–area principle, which determines discharge as the cross-sectional area multiplied by the mean velocity is applied to estimate the flowrate. Then the flowrate is used to calibrate the coefficients of the large flowmeters. The acoustic Doppler velocimeter is used to determine the velocities for determining the flow patterns in the open channel. Those velocities are used to calculate the maximum velocities and flowrates for determining the constant ratio of the mean to maximum velocity. The available data from the Taipei Water Department is used to illustrate the proposed method. The results show it can efficiently, accurately and reliably calibrate the large flowmeters in water distribution systems.