A calibration method for a liquid flowmeter comprising: providing a first tank (12) for receiving said liquid to be measured and a liquid storage tank (14) connected to said first tank (12); providing a liquid delivering line (9) for conveying the liquid from said liquid storage tank (14) to an external device; providing a weighing sensor (22) for weighing the liquid contained in either the first tank (12) or the liquid storage tank (14), wherein during performing the calibration for the flowmeter (20): disconnecting the liquid delivering line (9) in a conveyance direction downstream of the liquid flowmeter (20) from the external device; connecting said liquid delivering line (9) in the conveyance direction downstream of said liquid flowmeter (20) to a calibrating line (7) connected to said first tank (12); and determining a flow rate of the conveyed liquid within a predetermined time interval based on the change in the liquid weight measured by said weighing sensor (22) in said predetermined time interval, wherein the liquid from said liquid storage tank (14) is delivered through the liquid delivering line back to said first tank (12), and the flow rate of the liquid from the liquid storage tank (14) to said first tank (12) is controlled so as to be substantially constant during calibration.