A method for ascertaining an available fluid volume in a tank for brake fluid of a braking system. The braking system includes a pressure generator fluidically connected to the tank on the one hand and to at least one brake circuit on the other hand, which is activatable for generating a hydraulic pressure in the braking system as brake fluid is withdrawn from the tank. A drop below a predefined limiting value for a fill level of the brake fluid in the tank is monitored with the aid of a binary sensor assigned to the tank. It is provided that an actuation of the pressure generator is monitored and that the available fluid volume is ascertained as a function of the actuation of the pressure generator when the instantaneous fill level drops below the limiting value.