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 (14a, 15a) 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 (14a, 15a) 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.