Method for determining the size of a leak in a tank device containing a liquid on a vehicle, in particular a motor vehicle, wherein the liquid affects the pressure in the tank device as a result of evaporation, comprising the following steps: - generating a first pressure (p 1) in the tank device as the reference pressure at a first time (t 1), wherein ambient pressure (p 0) is generated as the first pressure; - recording a first resulting pressure curve to a second time (t 2); - generating a second pressure (p 4) at a third time (t 4), the first pressure (p 1) and the second pressure (p 4) being selected to be different, and creating a vacuum as the second pressure (p 4); - recording a second resulting pressure curve to a fourth time (t 5); - determining the pressure gradient of the first pressure curve to second time (t 2) and the pressure gradient of the second pressure curve to third time (t 4); - determining a first pressure difference (Δp 2) of the pressure from the reference pressure, at second time (t 2); - determining a second pressure difference (Δp 4) of the pressure from the reference pressure, at third time (t 4); - calculating the size of the leak as a function of the determined pressure gradients and pressure differences and the assumptions that the evaporation rate in the tank device is constant and that a leak rate results which is proportional to the root of the corresponding pressure difference, by means of the following formula: A = V p * α * ρ 2 * R * T * 1 Δp 4 * dp / dt 4 − dp / dt 2 1 + Δp 2 Δp 4, Where: V = volume of the tank device p = ambient pressure α = flow coefficient ρ = density of the gas in the tank device T = temperature of the gas R = gas constant ΔP 2 = pressure difference at the second time ΔP 4 = pressure difference at the third time (dp/dt) 2 = pressure gradient of the first pressure curve to the second time (dp/dt) 4 = pressure gradient of the second pressure curve to the third time - generating a first pressure (p 1) in the tank device as the reference pressure at a first time (t 1), wherein ambient pressure (p 0) is generated as the first pressure; - recording a first resulting pressure curve to a second time (t 2); - generating a second pressure (p 4) at a third time (t 4), the first pressure (p 1) and the second pressure (p 4) being selected to be different, and creating a vacuum as the second pressure (p 4); - recording a second resulting pressure curve to a fourth time (t 5); - determining the pressure gradient of the first pressure curve to second time (t 2) and the pressure gradient of the second pressure curve to third time (t 4); - determining a first pressure difference (Δp 2) of the pressure from the reference pressure, at second time (t 2); - determining a second pressure difference (Δp 4) of the pressure from the reference pressure, at third time (t 4); - calculating the size of the leak as a function of the determined pressure gradients and pressure differences and the assumptions that the evaporation rate in the tank device is constant and that a leak rate results which is proportional to the root of the corresponding pressure difference, by means of the following formula: A = V p * α * ρ 2 * R * T * 1 Δp 4 * dp / dt 4 − dp / dt 2 1 + Δp 2 Δp 4, Where: V = volume of the tank device p = ambient pressure α = flow coefficient ρ = density of the gas in the tank device T = temperature of the gas R = gas constant ΔP 2 = pressure difference at the second time ΔP 4 = pressure difference at the third time (dp/dt) 2 = pressure gradient of the first pressure curve to the second time (dp/dt) 4 = pressure gradient of the second pressure curve to the third time V = volume of the tank device p = ambient pressure α = flow coefficient ρ = density of the gas in the tank device T = temperature of the gas R = gas constant ΔP 2 = pressure difference at the second time ΔP 4 = pressure difference at the third time (dp/dt) 2 = pressure gradient of the first pressure curve to the second time (dp/dt) 4 = pressure gradient of the second pressure curve to the third time