Abstract: A design method for an air-conditioning/refrigeration circuit having a refrigerant with an oil portion, wherein at the design stage in a calculation of the air-conditioning/refrigeration circuit the fluid-dynamic and thermodynamic influence of the oil portion is approximated for at least one circuit component of the air-conditioning/refrigeration circuit only with refrigerant by a change of the refrigerant mass flow flowing through the circuit component. A reduction or increase is simulated by way of a bypass by which a bypass portion of the refrigerant mass flow is extracted from the circuit at an extraction point upstream of the circuit component, is made to bypass the circuit component and is then fed back to the circuit at a feed point.