A portable, personal electronic device for the body weight management of a user thereof, comprising: a control unit for controlling the device; and a user interface coupled to the control unit for entering input information and for obtaining output information, the control unit being configured to receive an initial weight, a target weight and food intake amount of the user via the user interface, to generate the energy expenditure of the user, to generate food intake amount and exercise instructions based on the difference between the user's initial weight and his target weight for display via the user interface, to receive the user's current weight via the user interface, and to generate a change in the user's weight, characterized in that the device further comprises an activity unit coupled to the control unit and comprising an acceleration sensor measuring the physical activity of the user, the control unit being configured to take into account the user's physical activity when generating the user's energy expenditure, and the control unit is further configured to calibrate itself, if the change in the user's weight does not correspond to the estimate made on the basis of his food intake amount and his energy expenditure, by eliminating the impact of errors caused by the user including incorrect input data entered by the user, and/or caused by errors appearing in default settings made by the device including levels of input data and conclusions drawn from these, so that the food intake amount and/or exercise instructions to be displayed via the user interface are modified such that if the change in the user's weight is smaller than estimated on the basis of the user's food intake amount and his energy expenditure, the control unit is configured to reduce the energy content of the food intake amount instructions and/or to increase the energy expenditure of the exercise instructions, or if the change in the user's weight is greater than estimated on the basis of the user's food intake amount and his energy expenditure, the control unit is configured to increase the energy content of the food intake amount instructions and/or to reduce the energy expenditure of the exercise instructions. a control unit for controlling the device; and a user interface coupled to the control unit for entering input information and for obtaining output information, the control unit being configured to receive an initial weight, a target weight and food intake amount of the user via the user interface, to generate the energy expenditure of the user, to generate food intake amount and exercise instructions based on the difference between the user's initial weight and his target weight for display via the user interface, to receive the user's current weight via the user interface, and to generate a change in the user's weight, characterized in that the device further comprises an activity unit coupled to the control unit and comprising an acceleration sensor measuring the physical activity of the user, the control unit being configured to take into account the user's physical activity when generating the user's energy expenditure, and the control unit is further configured to calibrate itself, if the change in the user's weight does not correspond to the estimate made on the basis of his food intake amount and his energy expenditure, by eliminating the impact of errors caused by the user including incorrect input data entered by the user, and/or caused by errors appearing in default settings made by the device including levels of input data and conclusions drawn from these, so that the food intake amount and/or exercise instructions to be displayed via the user interface are modified such that if the change in the user's weight is smaller than estimated on the basis of the user's food intake amount and his energy expenditure, the control unit is configured to reduce the energy content of the food intake amount instructions and/or to increase the energy expenditure of the exercise instructions, or if the change in the user's weight is greater than estimated on the basis of the user's food intake amount and his energy expenditure, the control unit is configured to increase the energy content of the food intake amount instructions and/or to reduce the energy expenditure of the exercise instructions.