Taking the mass of a sample is a routine operation in laboratories which is frequently repeated during data collection. In chemical industries, this operation is often done manually. A procedure for fully automatic weighing of liquid substances with a laboratory robot is presented. To demonstrate its functionality and a possible application, the laboratory robot was used to create test samples for calibration of a gas chromatograph. The achieved accuracy is greater than that of careful manual weighing. The presented method facilitates the handling of most liquids commonly used in laboratories.