The first challenge to properly size a vehicle A/C system is to define the vehicle air conditioning heat load requirement. Within automotive industry, a model to accurately define vehicle heat load is still under development. In this study, a simple method to calculate vehicle heat load is developed. The cooling load temperature differential (CLTD) method is used to calculate the heat gain of a sunlit roof and wall (door). This is done in one step by using ASHRAE data. The calculation presented here takes into account the geometrical configuration of the vehicle compartment including glazing surfaces (shading), windshield and roof angle, and vehicle orientation, Special attention is given to the calculation of direct and diffuse incidence solar radiation through the windshield and skylight glass. The vertical glass' solar heat gain is evaluated by using ASHRAE data. The U value method is used to calculate heat transfer between the outside and inside cabin. Heat gains from infiltration, occupant, and HVAC unit blower motors are considered in the cooling load calculation. The method accuracy was validated using wind tunnel tests. The results showed the predicted cooling load is very close to the tested value, and the deviation between calculated and tested heat loads is smaller with fresh air mode than that with recirculation mode.