Method for determining a liquid level in a tank for a liquid using an ultrasonic sensor arranged at the bottom of the tank for emitting and receiving an ultrasonic wave, and a first reflector arranged vertically above the ultrasonic sensor for reflecting an ultrasonic wave emitted from the ultrasonic sensor back to the ultrasonic sensor, and a second , a third and a fourth reflector arranged above the ultrasonic sensor and arranged below the first reflector , said second reflector arranged vertically above the ultrasonic sensor to deflect an ultrasonic wave emitted by the ultrasonic sensor to said third and said fourth reflector , and said third and said fourth reflector arranged at the same distance from said second reflector , and wherein the third reflector is arranged to reflect an ultrasonic wave incoming from the second reflector back to said second reflector , and wherein the fourth reflector is arranged deflect an ultrasonic wave incoming from said second reflector to the liquid level, the method comprising at least the following steps: a) determining a first transition time of an ultrasonic wave between the ultrasonic sensor and the first reflector , and b) determining a propagation speed of an ultrasonic wave in the liquid based the distance from the ultrasonic sensor via the second reflector to the third reflector and based on a respective measured second transition time; and c) determining a third transition time of an ultrasonic wave for the distance from the ultrasonic sensor via the second and fourth reflector to the liquid level; and d) calculating a liquid level based on the determined propagation speed of an ultrasonic wave in the liquid and based on the second and third measured transition times. a first reflector arranged vertically above the ultrasonic sensor for reflecting an ultrasonic wave emitted from the ultrasonic sensor back to the ultrasonic sensor, and a second , a third and a fourth reflector arranged above the ultrasonic sensor and arranged below the first reflector , said second reflector arranged vertically above the ultrasonic sensor to deflect an ultrasonic wave emitted by the ultrasonic sensor to said third and said fourth reflector , and said third and said fourth reflector arranged at the same distance from said second reflector , and wherein the third reflector is arranged to reflect an ultrasonic wave incoming from the second reflector back to said second reflector , and wherein the fourth reflector is arranged deflect an ultrasonic wave incoming from said second reflector to the liquid level, a) determining a first transition time of an ultrasonic wave between the ultrasonic sensor and the first reflector , and b) determining a propagation speed of an ultrasonic wave in the liquid based the distance from the ultrasonic sensor via the second reflector to the third reflector and based on a respective measured second transition time; and c) determining a third transition time of an ultrasonic wave for the distance from the ultrasonic sensor via the second and fourth reflector to the liquid level; and d) calculating a liquid level based on the determined propagation speed of an ultrasonic wave in the liquid and based on the second and third measured transition times. <EOS>