Method for estimating the quantity of fuel actually added (V r) when filling up a vehicle tank, carried out by a device comprising computer means connected to the sensors that are present on the vehicle, in particular a sensor for measuring the quantity of fuel in the tank, said estimation comprising at least the following steps: i) a step of estimating (Evrmax) a volume (Vr max) of fuel that can be put into the tank, said volume (Vr max) being estimated using a formula of the type: Vr max = V c + N p − N a where (V c) is the hidden capacity of the tank, (N p) is the plateau level of fuel and (N a) is the fuel level before filling up, the levels, the volume Vr max and the hidden capacity V c being expressed in litres; ii) a step of determining (Dp) whether a complete fill-up (Pc) or a partial fill-up (Pp) is being carried out; iii) a step of estimating (Evr) the quantity of fuel actually added (V r), given by a formula of the type: N p − N a ≤ V r ≤ V c + N p − N a for the case of a complete fill-up (Pc) and by a formula of the type: V r = N r − N a for the case of a partial fill-up (Pp), where Nr is the new level of fuel measured in the tank after the partial fill-up (Pp) expressed in litres, and characterized in that the step of estimating (Evrmax) the volume (Vr max) of fuel that can be put into the tank comprises a sub-step of estimating (Evc) the hidden capacity (V c) of the tank, said sub-step of estimating (Evc) the hidden capacity (V c) of the tank being carried out by combining a theoretical reduction in the level of fuel with an observed fuel level (N p) and a consumption phase which follows the plateau phase, with the aid of algorithms for estimating the consumption, taking into account the speed of the vehicle, the engine speed, the altitude, the fuel load and the characteristics of the vehicle. i) a step of estimating (Evrmax) a volume (Vr max) of fuel that can be put into the tank, said volume (Vr max) being estimated using a formula of the type: Vr max = V c + N p − N a where (V c) is the hidden capacity of the tank, (N p) is the plateau level of fuel and (N a) is the fuel level before filling up, the levels, the volume Vr max and the hidden capacity V c being expressed in litres; ii) a step of determining (Dp) whether a complete fill-up (Pc) or a partial fill-up (Pp) is being carried out; iii) a step of estimating (Evr) the quantity of fuel actually added (V r), given by a formula of the type: N p − N a ≤ V r ≤ V c + N p − N a for the case of a complete fill-up (Pc) and by a formula of the type: V r = N r − N a for the case of a partial fill-up (Pp), where Nr is the new level of fuel measured in the tank after the partial fill-up (Pp) expressed in litres, and characterized in that the step of estimating (Evrmax) the volume (Vr max) of fuel that can be put into the tank comprises a sub-step of estimating (Evc) the hidden capacity (V c) of the tank, said sub-step of estimating (Evc) the hidden capacity (V c) of the tank being carried out by combining a theoretical reduction in the level of fuel with an observed fuel level (N p) and a consumption phase which follows the plateau phase, with the aid of algorithms for estimating the consumption, taking into account the speed of the vehicle, the engine speed, the altitude, the fuel load and the characteristics of the vehicle. for the case of a partial fill-up (Pp), where Nr is the new level of fuel measured in the tank after the partial fill-up (Pp) expressed in litres, and characterized in that the step of estimating (Evrmax) the volume (Vr max) of fuel that can be put into the tank comprises a sub-step of estimating (Evc) the hidden capacity (V c) of the tank, said sub-step of estimating (Evc) the hidden capacity (V c) of the tank being carried out by combining a theoretical reduction in the level of fuel with an observed fuel level (N p) and a consumption phase which follows the plateau phase, with the aid of algorithms for estimating the consumption, taking into account the speed of the vehicle, the engine speed, the altitude, the fuel load and the characteristics of the vehicle.