1. A method of determining a flow rate (Q) produced by a pump controlled with a frequency converter, which produces estimates for rotational speed and torque of the pump, wherein a QH curve and QP curve of the pump are known, the method performed by a processor configured to: determine a shape of the QH curve of the pump; divide the QH curve into two or more regions depending on the shape of the OH curve; determine an operating region of the QH curve on which the pump is operating; and determine the flow rate (Q) of the pump using the determined operating region of the QH curve, wherein determining the operating region comprises: measuring a head produced by the pump; estimating the flow rate using a QP curve-based method; determining the operating region from the estimated flow rate wherein determining the flow rate uses the determined operating region of the QH curve and the measured head produced by the pump; and controlling the pump based on the determined operating region.