This paper describes a method for evaluating the equivalent temperature in vehicle cabins based on the new international standard ISO 14505-4, published in 2021. ISO 14505-4 defines two simulation methods to determine a thermal comfort index “equivalent temperature.” One method uses a numerical thermal manikin, and the other uses thermal factors to calculate. This study discusses the latter method to validate its accuracy, identify the key points to consider, and examine its advantages and disadvantages. First, the definition of equivalent temperature and the equation to calculate the equivalent temperature using thermal factors, such as air temperature, radiant temperature, solar radiation, and air velocity, are explained. In addition, the experiments and simulation methods are described. In the experiments, the equivalent temperature was measured by a thermal manikin in a vehicle cabin under two conditions: a summer cooling condition with solar radiation and a winter heating condition without solar radiation. Subsequently, the thermal factors in the vehicle cabin under the experimental conditions were simulated using CFD. Next, the equivalent temperature was calculated using the thermal factors. Finally, the calculated equivalent temperatures are compared with those measured in the experiments and those calculated by the numerical thermal manikin method mentioned above. The results showed a good correlation at most segments, with approximately 4°C difference, indicating that thermal sensation can be predicted within an accuracy of 1 point on the ASHRAE 7-point scale. Air velocity treatment is critical for the segments where direct airflow hits, to accurately evaluate the equivalent temperature at each body segment. This method can also be used with 0D/1D tools such as thermal management simulations because it does not always require 3D models. In addition, this method can be easily applied to parametric studies.