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 characteristic curve of the pump is known, the method comprising: determining a shape of a characteristic curve of the pump; dividing the characteristic curve into two or more regions depending on the shape of the characteristic curve; determining an operating region of the characteristic curve on which the pump is operating; and determining the flow rate (Q) of the pump using the determined operating region of the characteristic curve, wherein the characteristic curve is at least one of a flow rate to head (QH) curve or a pressure to flow rate curve.