The calibrating breathalyzer comprises an alcohol sensor, a non-volatile memory, a processing unit or processor, a display and a housing to house these components. The processing unit can calibrate the breathalyzer using the user's body as a simulator based on the user's metabolism rate, type and amount of alcohol consumed by the user. The processing unit determines a sample time to receive a breath sample from the user based on a time to a predetermined calibration point from the drinking start time calculated using the user's metabolism rate and the determined maximum alcohol level. The BAC % measurement based on the user's breath sample at the sample time is used as a reference point in calibrating the breathalyzer.