A calibrating breathalyzer apparatus, comprising: an alcohol sensor for receiving a breath air sample and measuring percent blood alcohol concentration (BAC%) based on analysis of the breath sample; characterized in that it further comprises: a non-volatile memory for storing calibration data of the alcohol sensor comprising one or more reference points; a processing unit for operating the breathalyzer apparatus in two modes: a calibration mode to calibrate the breathalyzer apparatus using the user's body as a simulator by receiving information regarding a metabolism rate of the user, type and amount of alcohol consumed by the user, and a drinking start time; determining maximum alcohol level from the type and amount of the alcohol consumed by the user; determining 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 metabolism rate of the user and the maximum alcohol level; receiving a BAC% measurement from the alcohol sensor based on the breath sample provided by the user at the sample time to provide a reference point; and storing the reference point; and an operation mode to provide a BAC% reading based on the BAC% measurement by the alcohol sensor and the calibration data stored in the non-volatile memory; a display for displaying the BAC% reading and other information to a user; and a housing to house the processing unit, the alcohol sensor, the non-volatile memory and the display. an alcohol sensor for receiving a breath air sample and measuring percent blood alcohol concentration (BAC%) based on analysis of the breath sample; characterized in that it further comprises: a non-volatile memory for storing calibration data of the alcohol sensor comprising one or more reference points; a processing unit for operating the breathalyzer apparatus in two modes: a calibration mode to calibrate the breathalyzer apparatus using the user's body as a simulator by receiving information regarding a metabolism rate of the user, type and amount of alcohol consumed by the user, and a drinking start time; determining maximum alcohol level from the type and amount of the alcohol consumed by the user; determining 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 metabolism rate of the user and the maximum alcohol level; receiving a BAC% measurement from the alcohol sensor based on the breath sample provided by the user at the sample time to provide a reference point; and storing the reference point; and an operation mode to provide a BAC% reading based on the BAC% measurement by the alcohol sensor and the calibration data stored in the non-volatile memory; a display for displaying the BAC% reading and other information to a user; and a housing to house the processing unit, the alcohol sensor, the non-volatile memory and the display. a non-volatile memory for storing calibration data of the alcohol sensor comprising one or more reference points; a processing unit for operating the breathalyzer apparatus in two modes: a calibration mode to calibrate the breathalyzer apparatus using the user's body as a simulator by receiving information regarding a metabolism rate of the user, type and amount of alcohol consumed by the user, and a drinking start time; determining maximum alcohol level from the type and amount of the alcohol consumed by the user; determining 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 metabolism rate of the user and the maximum alcohol level; receiving a BAC% measurement from the alcohol sensor based on the breath sample provided by the user at the sample time to provide a reference point; and storing the reference point; and an operation mode to provide a BAC% reading based on the BAC% measurement by the alcohol sensor and the calibration data stored in the non-volatile memory; a display for displaying the BAC% reading and other information to a user; and a housing to house the processing unit, the alcohol sensor, the non-volatile memory and the display.