Embodiments include a blood collecting system comprising a scale, a blood agitating system, a blood bag and a tube for extracting blood from a donor. The collecting system comprises, or is connected to, a processor for controlling the scale, the blood agitating system and the amount of blood being extracted from the donor. The processor is arranged to store one or more formulas for calculating an optimum amount of blood that can be donated. The processor can receive information regarding which formula to be used, the parameters necessary for the chosen formula. The processor is arranged to process the parameters in the chosen formula to calculate the amount of blood to be extracted. The processor is arranged to use the result of the calculation to control the collecting system to extract the calculated amount of blood and stop when the scale has detected that the calculated amount has been extracted.
