The invention concerns the automatic determination of an emission value for a motor vehicle. For this purpose, a system for the automatic determination of an emission value for a motor vehicle 4 is described that comprises the following: a fuel quantity sensor 1, an identification device 3 for registering an account identification, an account database 7 containing at least one account information item for each of a multitude of accounts 8, and a determination device 6, where: the fuel quantity sensor 1 determines a fuel quantity added to the tank and transmits it to the determination device 6; an account identification registered by the identification device 3 is transmitted to the determination device 6 and is associated with the fuel quantity added to the tank; and where the determination device 6 determines the emission value on the basis of the fuel quantity added to the tank and at least one account information item of the account 8 associated with the account identification. This permits a simple and reliable determination of the emission value that also meets with a high degree of acceptance by the users.