A method operating a refrigeration system with a heat pump function for a motor vehicle with an at least partially electric drive. The refrigeration system has a refrigerant compressor; a first heat exchanger; at least one further heat exchanger; a sensor device arranged upstream of the refrigerant compressor on the low-pressure side for detecting a first refrigerant pressure and/or a first refrigerant temperature; a sensor device arranged downstream of the refrigerant compressor on the high-pressure side for detecting a second refrigerant pressure and/or a second refrigerant temperature. The method includes detecting the first refrigerant pressure and/or the first refrigerant temperature; detecting the second refrigerant pressure and/or the second refrigerant temperature; detecting a rotational speed of the refrigerant compressor; determining a refrigerant volumetric efficiency; and determining a refrigerant mass flow based on the refrigerant volumetric efficiency.