A method for determining the size of a leak in the liquid-containing tank device of a vehicle, in particular of a motor vehicle, the liquid influencing the pressure in the tank device by evaporation, with the following steps: generating a first pressure as a reference pressure in the tank device at a first instant, detecting a first pressure characteristic up to a second instant, generating a second pressure at a third instant, the first pressure and the second pressure being chosen to be different, detecting a second pressure characteristic up to the fourth instant, determining the pressure gradient of the first pressure characteristic at the second instant and of the pressure gradient of the second pressure characteristic at the third instant, determining the first pressure difference of the pressure at the second instant from the reference pressure, determining the second pressure difference of the pressure at the third instant from the reference pressure, computing the size of the leak depending on the determined pressure gradient and the pressure differences, and the assumption that the evaporation rate is constant in the tank device, and that a leak rate is established which is proportional to the square root of the respective pressure difference.