Seat belt sensor is used for monitoring whether or not a seat belt is fastened. The sensor includes a load cell with a strain gage that is integrated into a seat belt mechanism that is used to secure an occupant to a vehicle seat. When the seat belt is tightened, the sensor is pulled into tension and this is measured by the strain gage. The strain gage measurements and signals are sent to the ECU. The ECU uses the information to determine whether a child seat or an adult is secured to the vehicle seat. Further, the seat belt force information is used to adjust the initial seat occupant weight and is used to properly classify the seat occupant.